OMG, that is absolutely not on. Your poor DD (and you).

Going to the principal is a great idea, does he/she even know about the 'sucking issue"? If they don't, it might be helpful for it to be brought to their attention, and if they do know, why did they not contact you about it?

Have these boys parents been informed of their behaviour?
What is the school going to do to ensure this doesnt happen again, (as it sounds like it has and will be an ongoing issue)

goodluck, hope you can sort this out quickly.

Nic