In the district here, you get suspended for having a knife, and expelled for threatening or showing the blade. You can not go back to the school, if you are expelled for that reason. To me, it makes sense that if he were violent or threatening with it, he would not be allowed back in the school.
If they did give it back...they probably did so, because they had not seen him use it inappropriately. It seems like most young guys have a pocketknife. I was in high school when Columbine happened. I carried a small Swiss Army knife on my key chain. I didn't even think about it, and was suspended. My parents had no problem getting me another. I used it responsibly, but just forgot that day.
Oh, and the school could have confiscated the knife and never gave it back. I don't actually think it's abnormal for a 12 year old to have a pocket knife, or to take it to school with no intentions. I definitely think IF did this, but the knife thing only matters now, and after the fact. It could have meant nothing at the time.