Do benzodiazepines (Xanax) work for severe OCD?
First of all, let me express my sympathy. I've been where you are (high school student with severe OCD) and I know how tough it is just to live with it from day to day. Kudos to you for making it this far.
How long have you been on the current antidepressant? For some classes of antidepressants (particularly SSRIs, which are most commonly used for OCD) it can take up to a month to see results. If it's only been a few weeks, I would try to stick with it at least until the one-month mark, just to see if it might help. On the other hand, if you have been on it for a long time (over a year) and it seems to have lost its efficiacy, you may have developed a tolerance. You could ask your psychiatrist about trying a different drug in the same class to see if it works any better.
Benzodiazepines in general are very sedating, so they're mostly helpful if the OCD is affecting your ability to relax and sleep in the evenings, after all your responsibilities have been taken care of. If this is your problem, instead of Xanax, you might want to ask about a less potent benzo such as Ativan (lorazepam). While it's still a benzo and as such has the potential to be habit-forming, this is much less of a concern than with a stronger benzo like Xanax, especially if you take it at a low (0.5–2 mg) dose. I use it myself a few times a week and while I have developed a slight tolerance, I haven't noticed that my anxiety is any worse on the days when I don't take it. It does have a pretty long half-life (around 12 hours), so be aware that you may still be feeling some residual effects in the morning if you take it too late at night. This is the case for most benzos, though.
In addition to medication, what, if any, therapeutic approaches have you tried? OCD (as you've found) is not terribly responsive to medication, so it usually helps to augment the meds with some form of therapy. I've found CBT (cognitive behavior therapy) very useful for myself, and I know others have had success with DBT (dialectical behavioral therapy). Your psychiatrist should be able to refer you to therapists in your area who provide these services.