sneak, snitch
[verb, noun]

1. to secretly tell someone in authority, especially a teacher, that someone else has done something bad, often in order to cause trouble
2. the person who does that
    

• She was always sneaking on other kids in the class.
• "You told Mrs Cooper that it was me who tipped the paint over, didn't you⁠—you nasty little sneak!"
• Eventually a classmate snitched, and Emma was called to the headteacher's office.
• If you keep snitching on your friends, you won’t have many left.

There is a huge difference between snitching and reporting! ✔