They make little jackets to cover this stuff up.
If nothing else...use a little sweater.
They won’t keep a sweater or jacket on and just post pone getting asked to leave. If this event was so important to the girls then they’d have read the rules and wore something that wasn’t an issue. It’s still a SCHOOL event and they have the right to a dress code that has been in effect for YEARS. Parents SHOULD be reading the handbook they sign in these rules, if it’s important to them ... .