Cops arrested an administrator at a Bay Ridge prep school for assaulting a 12-year-old student on Feb. 7.

Police say Adelphi Academy’s Jon Barry Kablack bruised the boy’s arm when he grabbed it while the child was on a field trip from the school on Ridge Boulevard between 85th and 86th streets.

Police were notified by the child’s mother after her son told her what happened that afternoon when he got home from school.

But the head of the private Adelphi Academy blasted the complaint as frivolous.

“This appears to be an act of retaliation aimed at the school by disgruntled parents,” Adelphi President Roy Blash said in a statement.

“While Adelphi Academy did receive an allegation from a parent regarding their young child, [we] fully and immediately investigated the matter, and by all accounts found the allegation to be baseless.”

Unlike public schools, private school administrators are not required by law to report alleged abuse to authorities, a spokesman for the state’s Department of Education said.

But the child’s family doesn’t plan to take this laying down: they have retained lawyer Bruce Baron to take legal action against the school.

Baron declined to comment.

Kablack has not been fired, school employees said.

“He took a couple of sick days, but, as far as I know, he’s not fired,” said Chip Cafiero, the school’s director of community and external relations, who defended Kablack.

“He is an excellent man,” Cafiero said.

Details about Kablack’s employment at the school could not be immediately confirmed, but Cafiero said that he was an assistant to the school’s director and “does a lot of odds and ends.”