I don't feel large numbers belong to gangs, but economic conditions are conducive for increased gang activity. Our school system has taken important steps to prevent that increase, but the schools can not defeat gangs alone. A concerted community effort is needed by everyone to reduce the potential of serious gang problems. Perhaps video cameras can be used in public areas of the school to monitor behavior. In the community you could enlist the aid of merchants to help identify gang members.
Identification is just one aspect of fighting gangs, but it's a big one.
Then you have to come up a plan to steer the kids away from gangs with alternative activities. Federal and state grants can help here.