All the salary structure and pay depends upon the selection that was carried upon by the school initially. They should pay you the salary where you were recruited for and same was accepted by you.
But, you can not deny to teach to primary when they are giving salary where you were recruited for. In case if, you are receiving the salary that is not as per your initial appointment terms, acceptance condition then that could become the case of harassment. If it is so, then you should submit your protest to the principal first and let him/her see it. You can wait for sometime and then you can write to school management OR to Government education department for resolution.
But, wait, they may expel you from school also as there are not much transparency in private school. So, think about it. You have very limited choice.