I absolutely agree with you and for that very same reason my teens (grades 9 and 11) are not allowed to have snapchat.
HOWEVER, we are also constantly in the very same quandry as you and quite frankly it's not quite as simple as suggesting the groups use a different app to message.
For some reason, whether or not it's logical or the best choice, snapchat is what kids are using for the group chats. And not even for bad reasons. In fact my daughter can't participate in her high school Christian fellowship group chat because it's on snapchat (unofficially, just all the kids in the group).
So far we've continued to say no. And quite frankly, it's just discluded them from these groups because realistically this is the app the kids are using. It's been ok. But It's an issue that constantly has to be revisited for us because it continues to come up.
We currently allow text, fb, and instagram for our high schoolers.
Last night ds (gr 11) asked again about snapchat because he joined a bball league and the kids use it to communicate.
I'm actually starting to debate allowing the kids to have it but limit it to the specific approved group conversations.
I don't know... So glad you brought it up.