Two high school hackers have been caught running a racket where they charged $5 to change fellow pupils' exam grades.

The South Florida Sun-Sentinel reported that the scam was rumbled when a teacher noticed that a zero she had given a student had changed to 100 per cent.

A quick check of other grades indicated that there had been similar tampering.

Two pupils have been suspended for 10 days following the incident at Western High School in Davie, Florida, but no criminal charges have been filed.

The attack has called into question the Broward County schools policy of keeping grades in a centralised computer database called the electronic grade book.

Assistant principal Monty Escabi believed that the students may have hacked into a computer to steal a teacher's password to the grade book.

He said that it is unknown how many grades were changed, but one student has admitted to altering 20 grades or attendance records for $5 a time.

The grades appeared to be for tests and quizzes rather than final exams, but teachers have been asked to review grades in previously saved documents for 4,500 pupils.

Escabi said that the students responsible for the attack were extremely computer literate.