CONCORD — An unnamed teenager was arrested this weekend in connection with an attack on a school district’s computer system that resulted in grade changes for a number of students, authorities said Thursday.

Police responded to a call April 25 from Mt. Diablo Unified School District staff about an apparent intrusion, and later learned that a teacher clicked on a “phishing” e-mail, sending her credentials to a hacker.

That hacker then used the credentials to log into the district’s computer system, changing grades for students. An investigation did not reveal any sign of stolen personally identifiable information.

With help from the U.S. Secret Service, the Contra Costa County district attorney’s office and Contra Costa County sheriff’s officials, a teenager was arrested Wednesday after a search by a K-9 officer’s “Dug,” trained in electronic-device detection, yielded an SD card hidden in a tissue box.

According to KGO-TV, Ygnacio Valley High School sophomore David Rotaro, 16, admitted to the hack, saying he had sent the e-mail to staff and used the retrieved username and password to raise or drop grades of 10 to 15 people, but not his own grades.

Rotaro, who was released from custody Wednesday, told the TV station “it was like stealing candy from a baby.”

Contact George Kelly at 408-859-5180.