And they'd lose as a team.
Really, besides Xavier (who Superman could speed-slap before he even considers using his powers), what could the X-Men's combined efforts do to actually slow, or even beat Superman?
And find kryptonite? Seriously, where is Beast gonna find kryptonite, which is extremely rare in the first place? And, what would he do when he came back to find the rest of his team laid out? Hell, what makes you think Superman would let Beast even get that far?
The only people who have once been x-men who coul deven bother superman:
Cable
Prof X??
Jean with full phoenix force
MAgneto
Magneto Clone (xorn imposter)
Joseph
Nate
Illyana (supes is weak against magic)
Other magic and psychic types might bother him for a billionth of a second, but their power is based on thought, and supes can move faster than the speed of thought.