Robinw, agree with sophiAmes - if the bag/container is a hip one, the other children might think your dd is really cool. The 7/8 year olds I know are really into accessories.
I don't know what else to suggest - could your dd try it out for a week and see what happens? If she gets teased then take it away and think again.
Does your dd have packed lunches? could the epi pen be kept with her lunch box? Then it would not stand out so much.
Hope get this sorted out and the school nurse doesn't upset you further.