No he doesn't...he will be eligible for the 2016 NBA draft
Everyone has assumed that, and I think you're right, but I guess you could make the argument that he won't be if he doesn't graduate from HS in June. The rule is 19 AND one year removed from HS graduation. A guy could leave HS without graduating, wait a year, and then enter the draft, but if Maker graduates after June, it's a legit question whether it pushes his eligibility back a year. He was initially in the class of 2016.
Interesting test case, like it would have been if Andre Drummond had stayed at a prep school for a 5th year, then tried to enter the draft (something he talked about doing before changing his mind). I suspect that Maker's people will successfully argue that he could have graduated in June, but that he needed some extra classes for college eligibility. The NBA seems far more interested in age than academic technicalities. Regardless, it would be a pretty unique situation. None of the recent guys who've changed classes have started college late.