How do you guys deal with your child if he or she starts making friends with the naughty or more troublesome kids in school?

It really depends on how troublesome these kids are -- if they are just loud and playful, I'd let it be and monitor my child, stepping in if I think there's something he shouldn't do. But if the kids are really naughty (getting into fights, bullying), I wouldn't have any qualms from forbidding my child from being friends with them.