I was prescribed benzos at around that age, maybe 2003 or 2004, and I've been taking them ever since. Xanax is one of the most habit forming and it can be difficult to remember what you were doing while on it. It can also cause people to act in ways that they otherwise wouldn't, which can lead to problems.
You've probably read up on benzos and xanax and may have used them before, I'm just telling you what I never get told when prescribed that stuff. I learned pretty quickly though, and I've never had any serious problems from using them, I've heard many stories though where people ended up in some pretty serious situations.
If you are anxious, a low dose may be able to help you with school. Try to keep the doses low as possible, try not to take every day and it would be best only take it short-term. I'm a bit surprised that your therapist didn't suggest something a bit milder than xanax though. If you have any concerns about taking it, talk to your therapist.
I agree with Klein, that it should probably only really be used for panic attacks.