It's a particle that means "until ~". 義務教育まで just means "until compulsory education". やる気がない者に構ってやるのは義務教育まで(だ)。 It's only until compulsory education that I care about people who aren't willing to do it. If you still have trouble parsing this sentence, check the grammar of cleft sentences (aka ~のは~だ construction).

Only top voted, non community-wiki answers of a minimum length are eligible