It is to dob somebody in. To dob somebody in.

就是 do someone in。 do someone in。

Just don't dob me in it, please!

别告发我 求求你！

You won't dob me in it, will you?

你不会告发我的 对吧？

I'd say, I'm gonna dob you in, which means I'm going to tell the teacher.

我会说， I'm gonna do you in，意思是我要告诉老师。