Reading High School graduates donned caps and gowns and marched in their graduation ceremony — but none of them got diplomas.

Officials in the southeastern Pennsylvania school withheld the diplomas of all 700 seniors Wednesday because some students hacked into the district's computer system to change their grades.

According to a staff memo, grades of 29 seniors were altered by students who logged onto the system as a teacher.

Principal Wynton Butler says officials discovered the problem Monday but didn't have time to check all student records to determine who was eligible to graduate. Instead, all the seniors were handed empty diploma folders.

The graduates will be able to pick up their diplomas Thursday in the school cafeteria.