I can see your point... her Mother does over-protect her, but I understand the kid's anxieties too. She's 11, the first amongst her group to get her period, and the bathrooms are the hangout for all the year 11 (16 year old) girls.
I think she's just a little intimidated to be around them.
I was the same way at 11 too... I think it's a fairly normal, and common anxiety to be wary of the "big kids" at school.