Generally one should submit his/her resignation letter before 1 month’s notice ( If you fail to submit it before 1 month, the School Authority may not pay you the last months salary). Don’t forget to take a receipt of it. If you have the receipt, you won’t face any problem in future. If the HM refuses to give you the receipt copy, you may send the resignation through Registered Post. Remember, in any circumstances, the school cannot restrain you to join the new post, provided you have any obligation or outstanding loan to the school. Thank you.