Snitch

Informal + disapproving : a person who tells someone in authority (such as the police or a teacher) about something wrong that someone has done: someone who snitches. Snitch is slang and is defined as stealing or tattling. ... The definition of a snitch is slang for a tattletale. An example of a snitch is a child who always tells when her friends do something wrong. Being a snitch is also being called a 6ix9ine because he snitched on fellow rappers in court. Don't be a snitch, you will have no friends and will never get a BF/GF, remember being a snitch is being a tattletale.

Bob told the teacher that Billy was playing games in class. Right there is where Bob snitched because he tattled on him. And Bob wouldn't really care if he played games anyway.