The city Department of Education is once again investigating a bad teacher for fixing grades — more than a decade after trying to fire him for falsifying Regents test scores, The Post has learned.

Critics say it’s a case study in what is wrong with a school system where tenured teachers are protected by their powerful union — even when they are failing children miserably.

In June 2011, Osman Abugana, now 63, was accused of “inappropriately changing the scores from failing to passing” on the state Regents physics exam for five students at Medgar Evers College Preparatory, a public middle-high school in Brooklyn.

“The deceit and dishonesty which the department finds to underlie this conduct are, in its view, moral failings which cannot be remediated,” according to a 2013 disciplinary op

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