Can you change your mind after having your resignation accepted and another teacher advertised for by the school?
Normally, no: otherwise schools wouldn't know where they were. However, if you have a reason, such as relocation because of a partner's employment falling through, ask the head where the process of seeking your replacement has reached. If applications haven't closed, the school might consider aborting the process. If your replacement has been offered and accepted a contract, you're out of luck. In future, don't resign until you are sure about your future employment.