A Bronx principal has been fined $1,000 for allowing a mayoral candidate to film a campaign ad inside his school last year.

The city’s Conflicts of Interest Board announced the penalty Thursday against Jamaal Bowman, principal of the Cornerstone Academy for Social Action Middle School, but did not identify the mayoral candidate.

The campaign Web site of long-shot Josh Thompson shows him inside a school with Bowman at his side.

Thompson had billed himself as an “education candidate” during his brief bid to unseat Mayor de Blasio in last year’s Democratic primary. He dropped out months before the election.

The City Charter doesn’t allow city facilities to be used for any political purposes.