D.B.
My son is a runner and gets these periodically especially when running long distances. The trainers pop the blister with a sterile needle to drain the fluid (you can dip it in alcohol or put it in a flame for a minute - but be careful, it gets hot!). The are careful to keep the covering skin intact. They then put some antibiotic cream on it and a pressure bandage so the skin doesn't slide around. Eventually the dead skin on top sloughs off and the new skin isn't so tender.
For gym I would let her wear the crocs to school and let the gym teacher insist that she put on sneakers just for that class. She'll listen to that teacher more than she listens to you. Send in a note to the gym teacher for a head's up. Let the teacher reinforce to the whole class (without singling out your daughter) that socks are important and so is gym!
Don't buy into the drama anymore - she brought this on herself and all you can say is "how unfortunate for you" and "now you know how to prevent it for next time." Put on the bandaid and walk away. This too shall pass.