That does sound odd to me; I had similar school-related issues (but after my junior year) but I was treated for Generalized Anxiety Disorder (GAD): http://psychcentral.com/disorders/sx24.htm

Are you seeing a therapist or just getting medication or how are they treating it? I would think with a thorough psych evaluation they'd have a clue, you must indeed have pointers to post traumatic stress but I would be like you and want an "event" I could point to.