Schools have found a couple of ways to collect RSVPs using ClassroomParent.
The most typical is to add a volunteer opportunity called "RSVP", or "I will attend", then use the Event communication tools to notify parents and collect their responses. Some schools will also add a "I cannot attend" opportunity, so parents can respond either way.
Another method that schools use is to create a $0 product in the store. The product is then included in the newsletter and parents can "purchase" the RSVP. This takes more lead time than using the event volunteer opportunity, but works better if you need to have individual RSVPs for each person in the family.