I'm thinking of stealing a flash drive from the school computer
So you are going to steal a flash drive from the school?
I had my own flash drive at the beginning of the semester, but I had a fight with someone who took it and lost it
It's wrong to take things that don't belong to you.  Even if someone else carelessly lost your property, it is still theft if you take something from the school, and you could get into a lot of trouble for it.
Thanks for the lecture. It's a stolen school material, It's going to get easy cash.
Well two wrongs are not going to make a right. It makes sense that you are upset about loosing your flash drive, however stealing another one isn't the fix. Maybe if you tell the school they may have an extra flash drive they can give you or let you borrow.