Matthew C. Beighey should get a lot of ironic extra credit points in his computer class, but will probably spend time in a juvenile detention facility instead, for writing an application to shut teachers out of their grading system. This was not the first time Matthew had been in trouble for computer-related mischief. Last Fall, he accessed school files containing Social Security numbers, driver's license numbers, home addresses, and other data on past and present transportation employees. Matthew's program logged into the grading system as a teacher and entered a false password three times, making it impossible for teachers to get into the system. "If I log on with an incorrect password three times, it locks me out," said District spokeswoman Kelly DeFeciani.