Boys are immature... compared to a girl. Girl's mature faster than boys. So, maybe those 12-year-old boys are actually mature but not as mature as girls.Here are some ways of dealing with those not-mature-yet boys:
Ignore them. If you ignore, they don't bother you.
Don't laugh at their immature jokes if you don't find them that funny
Don't encourage their behavior-they get fuel from your encouragement
Be positive-if you know they don't matter to you just smile and walk away
Be strong-they're more likely to make fun of you if you start to cry
In this case "showing them who's boss" isn't always the perfect way to go. They might either take it as a challenge or you might become viewed as bossy.
Take a deep breath-if your calm, he can't annoy you, right?
Focus on the tasks at hand-you don't have to pay attention to the boy!
If nothing else works or if he is really harassing you-Tell an adult you trust. There's always a solution!