One of four defendants in a highway rock-throwing incident that left an Ohio teacher severely injured is in jail after he opted to start his sentence before it's imposed.

A jail official said Monday that 19-year-old Brett Lahr spent the weekend behind bars in central Pennsylvania even though the judge hasn't yet accepted his plea of no contest to conspiracy to commit aggravated assault.

Lahr and three other teens were arrested last July after a nearly 5-pound rock was dropped from an overpass and crashed through the windshield of a car on Interstate 80, about 60 miles north of Harrisburg.

The rock landed on the head of Uniontown, Ohio, teacher Sharon Budd, who remains under 24-hour care and still needs additional surgery.

Lahr's lawyer wouldn't say why he entered jail early.