From the Marvel Database:Tits are too big for Jubilee. Also I think in the comics cannon, she was a legit D student. She had some disorder where she can't do math and wasn't really that great in other subjects anyway. I think that is a plot point in Xmen and gen x, about how she feels inferior to the new mutants and other gen x members when it comes to academics, whereas all the new mutants got Bs or higher in everything.
Jubilee suffers from Dyscalculia; a learning disability that causes severe difficulty in making arithmetical calculations. Given her status as an academic underachiever, this assessment can probably be assumed to have some degree of accuracy.