You may be out of luck. If no one is answering the phone during normal school hours right now, it likely means your highschool's office isn't staffed during the summer. You may be able to get a transcript through your school district (if you went to public school) so that's worth a try. The good news is (for you at least!) summer is almost over so there should be someone in your highschool's office soon. You can keep trying by phone or check out the school website to see if there's any information on the calendar about when they'll be back.