Grade 3 math, grade 6 math, how hard can they be ? It isn't as if they are taking calculus. Time was when you could do this stuff in your head. Since you can't change the kids, and they don't seem to be getting any better, it would be time to look at the curriculum and the methodology behind it. Certainly you don't need an advanced mathematics degree  to teach grade 6, although some posters have argued that. Accomplished musicians then for music classes, Olympic calibre athletes for phys-ed , recognized artists for art, english majors for reading and so on. Check the curriculum and as well check the text books.