Police said a 15-year-old Downingtown West High School student, whose name is being withheld because he is a minor, accessed private information, including names, addresses and Social Security numbers, of more than 50,000 people.
The student apparently used a flash drive to save the personal data of about 40,000 taxpayers and 15,000 students on May 15, investigators said.
The incident marked the second time a student has hacked into the school's system since December. The district said it had tightened up security since the late 2007 breach.
School officials sent a letter explaining the incident home with students and posted it to the district's Web site.
The letter stated that "the district doesn't believe the purpose of the breach was identity theft or to use any information acquired."
It also provided credit reporting agencies' contact information so people could monitor their accounts for suspicious activity.
The student has been released into his parents' custody.
The district plans to hold a public meeting next Thursday discussing cyber security.