The former “School of No” principal ousted after The Post exposed her excessive tardiness while her Queens school suffered is now suing for her job back — saying that because her union contract doesn’t actually specify a start time for her workday, she couldn’t be marked late.

An arbitrator rejected the excuse from Marcella Sills, saying it “defies common sense” because she was expected to show up when the students arrived by 8:30 a.m.

Sills was fired in February from her $128,000-a-year job as principal of PS 106 in Far Rockaway.

Her suit says the contract requires principals to work seven hours and 15 minutes a day, and doesn’t set a start and end time, according to a copy of the pact attached to the Manhattan civil suit.

“Without a start time there cannot be a late time,” Sills’ suit says, adding she had “no expectation, let alone obligation,” to show up before the bell rang.

Sills’ suit claims her ouster was due to “bias, excess of power and procedural defects,” and not because she was late for work 178 times between September 2012 and January 2014.

In the past, Sills blamed her tardiness on caring for her ailing mom.

PS 106 earned the nickname “School of No” because it had no books for the Common Core curriculum, no physical education or art classes, no nurse’s office and no special-ed teachers.

Instead, kids were put in the auditorium, where they watched “more movies than Siskel and Ebert,” a whistleblower told The Post.

The 16-year Department of Education veteran says her firing was too harsh a penalty because she had no prior disciplinary record.

“To have implemented termination — the harshest of punishments — in lieu of merely deducting from [her] salary is a penalty which shocks the conscious,” Sills says in the suit.

A city Law Department spokesman said, “It’s entirely proper to fire a principal who persistently fails to show up for work on time. We’ll review the complaint and respond accordingly.”