I stole a pen from my school

When i was in year 10, our teacher told us we needed pens for the test. I had no pens and my mum had forgotten to bring any into school so i picked a pen up and put it in my bag. When the test came round i took my pen out but the teacher said that was mine so i just put it back in my bag. i've never used that pen since. I've never admitted it either.