I accidentally stole somebody's panda usb in high school
Did you realize at the time that you were taking a usb that wasn't yours?
When I took it, I initially believed it to be mine.
When you say 'initially', that tells me you figured out it wasn't yours. It isn't right to keep items that don't belong to you, even if it was an accident.
I was in high school. It's in the past and it's no big deal.
Taking someone's USB drive is also taking important documents and files that are irreplaceable to them. Therefore it is much more serious for them then you may imagine. In the future it would be best to return something that you accidentally take.