In a high school lobby in New Jersey, the principal saw a student heading toward a stairway and moved to cut her off. There was physical contact between them, though no blows.

The interaction lasted less than a minute.

The student filed an affirmative action complaint against the principal, saying that he had grabbed her and “slammed” her against a wall. The student is Black; the principal is white and Latino.

The principal, reporting the episode later that day, said he was preventing an altercation between the student and three others, who said she had threatened them.

Over the months that followed, those roughly 60 seconds, captured partially on video, have divided neighbors across two towns, spawned two investigations and set off a legal process that could end with the principal in prison.

On March 11, almost exactly a year after the encounter, the principal, Frank Sanchez, was taken into custody and charged with assault and endangering a minor.

What happened that day last spring at Columbia High School, a high-performing school that serves the towns of Maplewood and South Orange, N.J., has become a Rorschach test for a liberal school district with a racially mixed population.