DON'T take it at school!!
Trust me, if you do I guarantee you'll be back on this thread saying yeah I wish I tried it at home first.
Opiates can be great for school, but as is the case with any drug, you want to make all options available to you when you use it the first time, because you don't know what you'll want to do on it.
You won't want to be stuck in a classroom not able to "let go" as you please... you'll want to sit on your couch with some reggae on bobbing your head with a giant grin on your face. (If anything, DON'T do it before gym... that's probably the worst thing you could do)
And opiates as your first drug choice should be very interesting, you'll have both the opiate barrier and the "drug induced euphoria" barrier broken at once, and trust me, your gonna love it

make sure to post back.
EDIT: Also, remember your first experience will be your most memorable one. Personally I took 10mg of oxies my first time and I'll never forget it, but I wish I took more.
15mg will work, you'll experience it for sure, it will be an awesome night. If 15 is all you have, it will def. work. But I would recommend more like 20-25mg if you can.