Nobody is forcing them to drop out of high school and join gangs. They have a free education, but cultural influences cause schooling to be far lower on the ladder of importance than it should be.
You seem to ignore many other factors in this statement. Many of the schools these kids go to are underfunded, making learning there difficult for the best of students, much less the mediocre ones. Moreover, many of them are in questionable economic positions - they may have the choice of staying in school, or trying to do something so their family has food, clothing, and a roof over their heads.
Maybe not "forced," but, "coerced," might be appropriate.
And remember, we are talking about *KIDS*. Not adults with large amounts of insight built up over years. The choices are being made by 12 to 15 year olds.