It can be. It's different these days...what once was considered child's play has taken a more serious turn. Did the girl willingly allow him or did he just do it on a whim? Either way he needs "the talk". Also at 14, it's a little old to be grabbing a girl's breast without permission. He's now old enough to know better.
Yes it's serious. If he will do that in the day light, in public, he could be dangerous. I suggest you tell your mom, and let her know you were worried, and didn't know what to do. She will understand. If the boy gets away with it, he won't stop there. It is against the law for anyone to touch or grab anyone on their body, especially on the private area. Tell your mom so it doesn't happen again.
Lot's of times we are embarrassed so we don't tell. Just remember this; it is not your fault, and you've done nothing wrong. Stand up for you rights to your body.
Good luck! You will do fine, after all, you had the sense to ask this question, so I think you are a bright person.
If some man did it to me and if I could resist the immediate urge to flatten him I might be inclined to call the police and press charges. I certainly would not let it "go". I would want to know if this was a pattern of escalating behavior or stupid teenage judgement. He needs to get the message loud and clear that nothing about what he did was ok. If he pulls a stunt like that again he may get charged with sexual assault. What may have started as goofing off went too far and he needs help with learning boundaries.
It depends if the girl is taking it seriously! Some girls at my school don't mind and let guys do inappropriate things like spank them! It could be sexual harassment I guess? He could get suspended! But I sure do hope he learns how to treat a lady!
I wouldn't let it go. I would talk to this child and make sure he understands personal space and boundaries. Also prepare the child for punishment because the girl has every right to press charges against him.