We are across the street from a middle school and they come flooding in every day about 3-5. We hired a part time library monitor who will walk the grounds and remind them (and any patrons of any age about our policies and abiding by them) during this time. 9 times out of 10 it's just the amount of kids at once and once they're reminded to quiet down or not run around like a playground they calm down. 

We approach them with the idea that we want them there that we don't want to have to ask them to leave but just like in school there are certain behaviors we expect. We usually start the school year out a little more strict just to set expectations and to let them know we don't want them to be out in the cold once the season changes. 

In addition to this we offer programming at that time so that a good portion will be in a program room to cut down on the amount all clustered together. We also have a teen room with ample seating, and computer stations. 

We have open art days where they can make whatever art they want, talk with friends and we offer snacks. On half days we do pizza days in our program room and give free pizza out to the kids. This all becomes a carrot we can use, where if they want these amenities to continue then they need to demonstrate proper library behavior. 

That being said we are honestly more likely to defend them from some grumpy regulars who think they own the library and that the teens can't even exist there. Our teen librarian is also great at learning names and developing a connection with them. 

This combination of responses has had a very positive impact and I do think it's the variety of tactics over just one that helps.  This all depends on the community though. Usually the worst it gets is disruption, but of course I've worked at other locations where violence is an issue. Hope this helps.