If a girl sneaked out of home at night to meet her boyfriend and a few other people already found out should that teen admit to her parents what she did?

Yes, you should, for a number of reasons. One reason is so they won't find out from someone else (and you know they will). If you tell them yourself what you did, then they should, hopefully, appreciate you owning up to it voluntarily. But you also have to realize that sneaking out to meet your boyfriend is going to possibly cause them to not trust you for a while. Since you are the one that broke the trust, then you are the one that will have to earn it back. And you have to be willing to accept whatever punishment they (may) decide to give you since this was not only a breach of their trust, but also a breach of their rules for you. When you tell them, and I hope you do, just let them know how sorry you are, and make sure you apologize for the right reasons.