The district will have records of all students who attended the closed high school. They will be able to provide you with a copy of your diploma or transcripts.
Contact the state department of education.
The department will have records of all high schools in the state, including those that have been closed. They may be able to provide you with a copy of your diploma or transcripts.
Contact a private records company.
There are companies that specialize in retrieving records from closed schools. These companies may be able to provide you with a copy of your diploma or transcripts.