I'd probably say "sneaky" or "tricky," but "sly" also works. Another option is "devious." I also agree with Alysson that "naughty" could work, too, but "naughty" doesn't quite capture the fact that the kid is using deception/trickery to do naughty things.
>>a kid who fakes a sickness to not go to school( or should I say "NOT TO GO TO SCHOOL"??)
Both are fine. :) In formal writing, you should say "not to go to school" (because it's best to avoid using "split infinitives" when possible). In normal speech, though, people will say it both ways.