Self defense and minors?

I'am 21 years old and a group of my friends and I went to the beach for a day of relaxation. While a group of 4 of us were in the lake swimming, two 13 year old girls were about 10 ft away. These two girls were constantly throwing with all their force medium to large rocks AT us. They tried to claim that they were skipping rocks with "bad aim", and after we asked them numerous times to stop...they started to throw more, larger rocks. We decided among the group that it would only be fair to do the same, but some nearby middle aged people warned us not to, bc if police were involved, automatically myself and my friends would get in trouble due to the fact that these girls were not 18. Eventually we all swam out so far that the two girls couldnt touch the bottom and they gave up. However my friends and i were wondering if one of us would have maybe snatched them up or something if that WOULD be considered self defense, or ualt on a minor. Some of the rocks they were throwing DID hit some of us, one even hit my boyfriend in the back of the head and scd him. We know that we could have called the cops, but in the town we live in we would have been waiting for over an hour for them to get to the scene, and by then.... the girls probably would have ran off somewhere.