First things first, 'tattle' is not unique to American English. It is perfectly understood in the UK.
But what exactly do you want to say? It is confusing.
As I understand it, you have a student who did something wrong in class. Another student in the class knew who committed the offence, and told you who it was. Is that correct?
'Rattled' is wrong, forget about it.
The problem with 'to rat', 'to tattle', 'to squeal' and 'to grass' is that they all carry negative connotations. They are not neutral language. Here we are dealing with the concept of 'honour amongst thieves', where informing on someone else's wrongdoing has historically been seen as a bad or dishonourable thing to do.
The verb 'to rat.' Think about it, you are comparing someone to a rat in their characteristics. For 'to squeal', the connotation is that they are like a squealing pig. I'm not sure of the etymology of 'to grass' but make no mistake it is not a neutral way to describe someone.
So it depends how you want to say it. Someone in a position of authority, like a teacher, like a policeman, someone who wants a student or a member of the public to do the right thing should not use any of those verbs. Just use neutral language, the student 'told' the teacher what had happened.