B.C.
Google "show choir short hair styles".
https://www.pinterest.com/zigglefritz/show-choir-hair/
If/when you find a style that would work for the girls, you'll have to clear it with who ever is in charge (who has the say in how they wear their hair) before they can run with the idea.
It's going to be tough to find one look that will flatter everybody but if they have to go for uniformity/cookie-cutter look then slicking it back might not be a bad option.
It's nothing that a lot of styling gel won't take care of.
They are performing as a group - performers eventually get over 'self-conscious' to a degree to develop self confidence - which is something in middle school that every kid is working on.
Make suggestions but once the person in charge makes a ruling, then pipe down and go with the flow.