A Brooklyn principal was found guilty of not reporting $1,500 in missing funds — but whether the money was stolen or who took it remains a mystery.

Lowell Coleman, 64, former principal of PS 181 in East Flatbush, failed to report cash disappearing on a regular basis from a safe and office, an arbitrator ruled in an administrative trial.

Two of Coleman’s subordinates tried to finger the principal, who made $140,000 a year. One accused him of using the fund-raising stash as his “personal ATM.”

Cash from various school fund-raisers was kept in small white envelopes instead of deposited in the school’s bank account. Bookkeeping, by treasurer Irwin Drucker, a teacher and dean, was “sloppy,” testimony found.

In an episode befitting the Keystone Cops, then-Assistant Principal George Patterson claimed he “set a trap” by putting $500 cash in his own desk drawer.Soon after, Patterson said, he found the money missing.

Patterson insisted that Coleman had taken the $500 because he didn’t report it.

Coleman, who denied taking any money, refused to blame anyone until he confirmed the loss.

“No one ever proved that any money was missing,” said Betsy Combier, a paralegal who helped in Coleman’s defense.

Patterson secretly taped several phone conversations withColeman in an effort to incriminate him, but the recordings were so garbled that hearing officer Joel Douglas tossed them out as evidence.

The city Department of Education sought to terminate Coleman, but Douglas said the principal’s otherwise trouble-free history did not warrant it. Instead, he demoted Coleman to an assistant principal, saying he could be reinstated as a principal in a year if he earned a satisfactory rating.

Instead, Coleman retired. Suspended without pay in November 2013, he will get none of the $200,000 in lost pay, the DOE said.