In high school, a classmate of mine since kindergarten, took a fountain pen I left on top of my notebook for the only time in my school life. It was recess and I had stepped out for a couple of minute to get a sip of water.
I found out about her guilt because she used it for an exam and it was shiny steel and had glittering faceted designs. The only on in the entire school because we lived in a tiny town and there was no Internet shopping in those days.
She was confronted after the exam and for weeks after that, even the involvements of parents did not do anything. She flat out denied taking it while refusing to tell us where she got it.
I should not have been surprised, a couple of years before, she and other classmates were regularly cheating on classroom tests, using textbooks, in no reference permitted exams.
So I feel your pain.