the effect of Xaiver's School states that "Activate >>> Ready target X-Men character you control with a cost of 3 or less. That character cannot attack this turn."

does that mean that i can ready a stunned x-men character that didn't attack this turn?

and can i ready characters that already attacked this turn as long as i don't attack with them again this turn?

You don't ready stunned characters, you recover them. So no, Xavier's School won't help with your stunned characters. But yes, you can ready a character that has already attacked.

wouldn't the card say " Ready target non-stunned X-Men character..." if you can't ready stunned x-men? personally, i think The Golden Rule applies here

If we were playing a game together, I'd be more than willing to let you ready your stunned character so long as you didn't try to turn it face up.

:)

Page 17 of the rulebook states:

Nothing on Xavier's School says it allows you to ready a stunned character. The same holds true for Cosmic Radiation, Doom-Bots or any other cards that allow you to ready a character. Unless it specifically says it allows you to ready a stunned character (and thereby invoking the Golden Rule), you cannot ready a stunned character.