>>93489574They're not all the same age. Lee is the oldest, Marie is the middle child, May is the youngest.
Momma Kanker probably got pregnant shortly after each child was born, so they're very close in age. As far as school goes, the show seemed to imply that May is actually pretty smart, so she's probably a grade ahead, Marie is probably in the same spot, and Lee was probably held back (not necessarily because she's dumb, Lee tends to be the more violent of the three so she probably got a lot of detentions and suspensions, leading to shitty grades).
