My permanent High School record was altered without my knowledge and the school refuses to correct the mistake. What can I do?

You need to take the correct records, and the altered records to the school and ask for a correction in writing.  Put together a letter showing what the record was and how it was changed and ask for an explanation of why it was changed.  SEnd that letter Certified Mail so that you have proof it was received.  Keep copies of all of your documentation.