"Il continue d'être un mauvais garçon."
Mauvais is an adjective modifying garçon. There is a useful acronym B.A.G.S. which stands for Beauty, Age, Goodness, and Size. If an adjective is in one of those categories (beau, juene, bonne, grande, etc.) it usually goes before the noun.
"Mauvais" is a measurement of goodness, so it goes before the noun garçon.
It's really more of a rule of thumb to help you guess if you have no idea of where the adjective should go.
French adjectives seem to be very fickle. As I understand it, there is more leeway to adjective placement than rigidly going before / after the nouns they modify... sometimes you can switch the order if you want to change the emphasis of the sentence, but really it's just something that just comes with practice. After a while adjectives will start to sound wrong if they're not at the right place.