I used to feel this way too. I hated my school assemblies (we had them twice a week usually) and I would panic to no end. When I got to college I almost skipped my own graduation because of this because I would have to sit on stage, with NO escape, but I made it!

The way I coped with assemblies was to sit with good friends and to ALWAYS sit on the end, and if I could, at the very back right near the door (or at the very front, so I couldn't see everyone). It helped to have a little plastic bag in backpack/schoolbag and a bottle of water as well. I also found it helpful to have something to fidget with so I could distract myself from the anxiety.