Me and my friends have been sitting in the same area (back of library.. got benches and a little pond) for years.. We have been searched SOOO many times for all sorts of bullshit excuses from drugs to weapons to car theft (because we're next to a carpark we must be car thieves)..
They don't need to know you or have a valid excuse.. they'll just make one up on the spot..
Me and a couple of friends have also miraculously perfectly matched the description of randoms such as "people climbing into peoples gardens" or "car thieves"..
Cops spend their whole working career talking to people who constantly are lying to them. Speeders lie, drunk drivers lie, people high on pot or other drugs lie, people who commit crimes of violence or theft lie, people who sell and use drugs lie, etc, etc.
So this skews their world view and they basically assume that you are lying to them because everyone lies to them, all the time.
They also assume you are up to no good if you fit the profile or behavior of teens who party. If your sitting on the grass in front of the library at 2pm on a Saturday afternoon reading your school books cops will most likely profile you as a good citizen, good student, etc. If you are wearing a leather jacket or a concert tee shirt and have long hair cops will assume your a pot smoking party dude.
So it's important to know how cops think so you can position yourself as much as possible to be profiled as an innocent citizen nerd or whatever rather than someone who likely parties in any way.
That's a lot of the battle right there.
You also want to be aware that if you fit any kind of profile that the cops would be interested in, you do not want to ever be in possession of any weapons or any drugs or alcohol while out in public. If you happen to be in possession of drugs you want to have them on your body (or IN your body) to minimize chances they will be found if searched. So when you go out and your into pot, you do NOT want to carry a bong or a pipe. You do NOT want to carry a baggie of weed. You only want to have the ammt your going to smoke that day on you, no more. And you want it hidden in a good place. It's unlikely that cops will find 2-3 joints on you if your smart about where you are carrying them. If you carry a 1/4 oz of weed it makes it easier for them to find the drugs.
Your better off carrying a book of matches than a lighter. This is particularly true if you smoke pot but do not smoke cigarettes. Cops spend their entire lives talking to people who constantly lie to them, they become pretty good at figuring out whether or not a person is lying to them. In fact they can be a royal pain in the ass because even if you are telling the complete truth (when your innocent) they are going to friggin assume your lying to them and they will keep asking you in annoying ways over and over when you tell them the truth because they assume your lying to them because that's all that anyone does, lie to them.
So be smart. Do not fit the profile of a person of interest. If your up to no good (anything a cop would think is no good) then you want to make sure that everyone involved has their stories straight. Don't be a schmuck who is doing some bullshit with their friend and then when cops split you up and ask you questions about what your up to you give one story and your friend gives a different story lol. This is so basic to me but yet tons of people get screwed up here because they didn't put in the necessary planning ahead of time.
If your ever in a situation where your up to no good and cops are asking you and your friends (separately) about what your doing, where your coming from, where your going, how you know each other, etc, etc. If you are not SURE that your friend is going to say the same things you are going to say you don't want to give the cops ANY STORY. This is because your story will obviously not match the story your friend gives. You must rehearse your stories ahead of time and go over it again and again AND also be sure that your friend will remember and use the story rehearsed. And be sure that your friend will stick to it regardless of what the cops say (for instance they play your friend saying that you said something other than the story you agreed upon) then you DO NOT tell the cops answers to their questions.
If your buying drugs off of someone who you do not know, or someone whose street name you know but not their real name, not their last name, etc then you don't want to tell them "It's my cousin" or "We go to school together" or ANYTHING like that. This is because the cops are going to quiz you more and more... like 'what school you go to' and 'how long have you known each other' and 'where he lives' and 'what grade are you both in' or A MILLION other questions they can quiz you on. And your lying so obviously your stories are not going to match. This gives the cops reasonable suspicion that your up to no good. Basically they know your up to no good, because they know your lying to them and if you were not up to no good you would not lie to them.
So what you do is NOT tell them any fucking stories!
Where do you know him from? "Officer, with all due respect, I do not want to answer any questions from any government employees or government officials"
The reason I suggest you say that type of thing rather than "Officer I do not wish to answer any questions" is because I feel that by *misdirecting* the cops, leading them to believe that you have some issues or contempt for government, rather than merely 'protecting and asserting your rights' will put you in a better position. This way the cops will not be thinking "this little punk is hiding something, or this little punk is being a smart-ass" or something like that.
What they will be thinking, most likely, is "this kid has some issues with government, this kid is a political nutcase, or this kid probably has a father than is a certified constitutional nutcase" or something like that.
I strongly prefer cops to think that kind of thing about me. This way they will not profile me as someone who has studied the law, studied their civil rights, been advised by a lawyer, etc.... Even though knowing and asserting your rights is perfectly legal, and SHOULD NOT make the cops suspicious of you, it will make them think that your doing something illegal as the reason your asserting your rights. I want them to think I'm basically a political extremest who is anti-government, and it just so happens that because I am anti-government I do not want to dialogue with the police because they represent or are employees of the government. If possible and it seems like a reasonable thing to do without making the cops think that I am intentionally kissing their asses because I'm hiding something I will also try to get in some comment like "Officer, it's nothing personal at all, I have the utmost respect for police officers for their bravery they show in the line of duty rushing to the scenes of violent crimes and risking their lives to protect citizens... I simply do not believe in government, I am an anarchist (make sure you know what an anarchist is and that your able to dialogue about why you don't believe in government if you take this approach) and it just so happens that your employed by the government. I respect you, your badge, and the selfless actions you display to protect the public, but I do not want to answer any questions by anyone representing the government" And you stick to that, keeping it short and sweet, re-asserting "I do not wish to answer questions from anyone employed by government officer".
Another benefit of taking this approach is that the police may believe that you and/or your parents, as political extremists, are much more likely than the average citizen to go to extensive lengths filing complaints with the police dept, etc because your extremists and won't let minor mis-steps made by police because of your political views. As such, the cop may be thinking "OMG, this guy is a whacko, I don't want him marching out in front of the police station holding a sign that says "Officer so and so stomped on my constitutional rights" (I'm exaggerating to make my point).
Cops also don't want to listen to some political extremist ramble about political beliefs that they do not hold (just like you and I don't wish to hear people go on and on with political views that are opposite from ours).
Another potential side benefit is that the cops are going to remember you as the political guy who respects cops heroic actions keeping the public safe but is a major political nut. This is much better than be remembered as "One of the kids who hangs out behind the library and likely smokes pot." (or whatever).
I did this kind of thing the last time I was questioned by cops and initially the cop was defensive (which is GREAT for me). When I rambled about 'not answering questions from government people, bla, bla... the cop was all like "I totally respect your views" or something like that in a defensive way, like he was nervous to be around me the way we are often nervous being interrogated by cops!!!!
And later I really won them over by talking about how i think cops are hero's for rushing to the scene's of violent crimes to protect people when everyone else is running away from the situation.... and I think that politicians are responsible for creating unnecessary tension between police and citizens by having police officers involved in routine traffic stops rather than leaving them alone to focus on being ready to respond to calls when a wife is being beaten... that stuff is hard enough and stressful enough.... police officers shouldn't be hassled with the duties of pulling over cars for minor traffic violations and stuff like that..... (feel free to use your own variation of this kind of stuff... practice saying it alone, out loud, or with friends so that when the time comes you can say it to cops and get them off your jock about maybe having a beer behind the library).
Sun Tzu, The Art of War. Every battle is won before it is ever fought."
Be prepared for encounters with the police so that you are confident you will win the battle with the police when questioned before it even starts.