Why is math his worst subject? Math has never require flexibility last I heard. It's more about putting in specific numbers/variables into specific equations in order to get specific answers. Writing should be his worst subject. Thats where you really have to think "flexibly".
So happy that the author refrained from using "GHOST TENTAcleS" or some similar shit in order to make the series Fanservice. A series like this with actual PLOT!