Sorry I didn't answer this question earlier, just spotted it now:
Some of the families may have made such rules, but they were never enforced. Sure, the Don may have made a decree that drugs shouldn't be sold near schools, but the Don never personally checked to make sure his order was being carried out. Nor, when his underlings delivered his cut, did he ever say, "Uh, by the way, is any of this money from school kids?" What might happen: If one of his people was caught selling drugs at a school, and the cops identified the guy as a member of his family, and it looked like the guy might get a stiff-enough sentence to make him squeal, then the Don would get involved. That's what happened in the case of Angelo (Quack-Quack) Ruggiero and Paul Castellano.