I had a similar thing with a pen, the very same year I think… I had a mildly special pen which one day I lost. Went looking for it and found it sitting on a (slightly older) classmate’s desk, so i grabbed it and said “hey, that’s mine”. He tried to pretend that no, it was his, and he sounded very convincing about it, and even got the teacher involved. They both looked at me with infuriatingly condescending expressions as I explained how it was mine.

The teacher suggested “just let him have it” to the classmate, who conceded.

I went back to my desk fuming and scratched my initials into it before returning to show them, "look, see, it was mine! The classmate immediately pointed out “you scratched those in just now” and I think I mumbled something incoherent before going back to my desk, to the teacher’s mortification with the whole situation.

It had already begun dawning on me at this point that the classmate was right… That wasn’t my pen. It was his and just looked like mine. But it was too late at this point and I didn’t know how to handle it other than to keep quiet and try to forget about it.