By Selim Algar

City officials placed a single metal detector in a Brooklyn school building the day after a student was busted with a loaded gun — and turned up 21 weapons in just one day, law-enforcement sources said.

The Department of Education installed the scanner in the complex housing the Urban Assembly School for Law and Justice and two other schools in Downtown Brooklyn Thursday morning after the 17-year-old was nabbed there allegedly with a 9mm pistol and more than $30,000 in cash.

The school does not normally have a scanner in place. And Thursday’s surprise installation netted a varied arsenal — nine knives, seven cans of pepper spray, four stun guns and a pair of brass knuckles, sources said.

The 17-year-old had been busted a day earlier after showing up late for school at around noon and entering an office to log his attendance. A school staffer spotted what she thought was a pistol in his backpack and alerted school safety agents.

The student was detained until NYPD units arrived and allegedly discovered the gun and the cash in his possession.

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