In a small courtroom north of Houston, a fourth-grader walked up to the bench with his mother. Too short to see the judge, he stood on a stool. He was dressed in a polo shirt and dark slacks on a sweltering summer morning.

“Guilty,” the boy’s mother heard him say.

He had been part of a scuffle on a school bus.

In another generation, he might have received only a scolding from the principal or a period of detention. But an array of get-tough policies in U.S. schools in the past two decades has brought many students into contact with police and courts — part of a trend some experts call the criminalization of student discipline.