No reason why she can't have lunch in a small group, so long as she doesn't see it as not being 'cool!'
Same with exams, a small room with only her or a few others who find the sensory overload too much.
She should be allowed de-stressers, just try to make them socially acceptable and not distracting for others, stress-ball, worry beads, something she'll like.
Will they do a homework club at school? My DS's school does/will do. if it's an extension of school, rather than you having to get her to do it?
Assemblies, hmm, good to try to get de-sensitised. MP3 earphones, even if not plugged into MP3? Sat near the door so she can leave if it gets too much? I'm sure there are other solutions.
Sports day, tempted to take a sickie, or an authorised 'sickie' for anxiety levels.
Same with mufti days, my DS hates them, (especially when I forget and send him in in uniform) 