Misbehaviour is the noun: the bad behaviour itself — the acting-up that other people will not accept. School letters, news, and formal talk like this spelling with -our (British); American English writes misbehavior. It is usually uncountable: we say “his misbehaviour,” not “a misbehaviour,” unless we add a word like “an act of misbehaviour.” It covers anything from a child talking in class to a public figure causing a scandal; the seriousness sits in the situation, not in the word.