Here's what I got from your post, your lack of punctuation makes it a bit confusing.
1.)You went into a store and tried to steal a drink worth $2
2.) The owner of the store took your backpack, hat, glasses, wallet, and the contents of your backpack.
3.) The owner kicked you out of the store without returning the backpack.
4.) The owner threatened to call the police, and/or your school.
Is that the correct order of events?
To your concerns, no, it is not legal for him to keep your backpack, and your other stuff. However it also wasn't legal for you to steal that drink. His wrong does not negate yours (nor does yours negate his).
The best way to handle this without your parents finding out is to call the store, speak to the owner politely, apologize, and ask for your stuff back. If he gives it back then you are all set. if not then you are out of luck.
If you just want your stuff back and the owner refuses then you can call the police and file a complaint, however you can expect the owner to file a complaint and you can expect to be arrested for shop lifting. You can also gather proof that the belongings are yours, and then sue the owner for them, however this will likely also lead to the owner filing a criminal complaint and you being arrested.
If any kind of legal action takes place then your parents will find out.

