Two US school students accused of hacking into their institution's computer for stealing test papers and changing grades, will appear in a Silicon Valley court on Tuesday.

Eighteen-year-olds Omar Khan and Tanvir Singh, students at Tesoro High School in southern California who were charged last month, are jointly facing 73 felony counts for altering public records, computer access and fraud, burglary, identity theft and conspiracy.

Their arraignment has been postponed twice already, most recently on July 8 when they hired new attorneys, and could be postponed again on Tuesday, said Susan Schroeder, official spokeswoman, Orange County District Attorney.

The charges against Omar include altering and stealing public records, computer fraud, burglary, identify theft and receiving stolen property and conspiracy.

Tanvir is charged with one count each of conspiracy, burglary, computer access and fraud and attempted altering of a public record.

In their burglary attempt in May, Omar and Tanvir, tried to steal a copy of a test that was to be administered the following day, authorities said.

The two were caught by a night custodian as they were accessing a school computer.

Prosecutors claim that Omar, who is alleged to have broken into the school late at night with a stolen master key, also changed the grades of 12 other students, and that he installed spyware on school hard drives that allowed him to access the computers from remote locations.

Tesoro High School, with 2,800 students, is often ranked as one of the country's best. The district attorney's office is still investigating whether more students were involved, Schroeder said, and more arrests could be made.