Find the s.o.b. who actually stole it and the nail them to the wall.
You can't, you are now 'marked' for life. So what was it anyway?
If you really didn't steal it, and it is not in your possession then you need to find evidence of who really could have stolen it. You can only prove you didn't do something by proving that somebody else did it. The item might not even have been stolen at all but "misplaced" by the owner or put in a different place by somebody else and now it can't be found so they are assuming it has been stolen. I hope you get to be proved innocent if you really are. Good luck. God is the real witness to prove who stole it. The right person will be punished by God accordingly.
It depends if you actually have proof or not.
Then just NEVER bow down and say that you did it. Say "no" the whole way.
Well, find the thief and if you can't just let them talk until they are satisfied and let life do the course.
Usually somekind and somehow people will eventually knowmthe truth
Find the person who DID steal the item.
That IS a difficult situation and extravagantly unfair on you. I wonder if the theif was the girl who said she saw you put something in your pocket?
If the headmaster is wise, he'll work it out. He might see through them. Especially if they are known thieves or otherwise do the wrong thing.
You are innocent until proven guilty. The headmaster may do something like, threaten if the person who stole, doesn't own up then everyone gets punished in some way - some incentive like that ..whether it works or not is another thing. If he knows you are a good person who doesn't steal, he would be less likely to suspect you.
They should not punish everyone, only threaten to do so in order to get the person who did it to own up. If everyone knows everybody and everything, then someone may tell the headmaster who did it. He should only punish the person who did it. That is all that should happen. There is nothing worse than the injustice of being accused of something you didn't do, I know. But hang in there and trust that justice will be done.