HARRISBURG (AP) -- 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 serving time before his sentence has been determined. A jail official said Monday that Brett Lahr, 19, spent the weekend behind bars in central Pennsylvania even though the judge has not yet accepted his plea of no contest to conspiracy to commit aggravated assault. Lahr's attorney, Brian Manchester, said sentencing guidelines call for a minimum of 12 to 18 months and a maximum of 10 to 20 years. He declined to discuss why Lahr opted to begin jail time early. "From what his role was, from what I know, the sentencing seems appropriate," said Randy Budd, whose wife, Sharon, remains under around-the-clock care and faces additional surgery. Lahr and three other teens, including brother Dylan Lahr, 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 directly on Sharon Budd, a resident of Uniontown, Ohio, who was driving through Pennsylvania on her way to a show in New York. Budd taught language arts at Edison Middle School in Massillon, Ohio. Union County District Attorney Peter Johnson said Monday that defendant Keefer McGee, who testified against Brett Lahr at a preliminary hearing, is still cooperating with the prosecution. McGee's deal could result in a sentence of less than two years, which would allow him to serve it in the county jail rather than a state prison. McGee testified in August that he, the Lahr brothers and Tyler Porter had planned to cause some damage but did not expect their actions would result in the injuries that Sharon Budd suffered. He told the court that Porter and Dylan Lahr got out of his car at the overpass, armed with rocks they had collected earlier. "I knew they were going to throw rocks, but never thought they were going to hurt anyone as they did it," he testified last year. A hearing is scheduled for later this month on efforts by McGee, 18, of New Columbia; Porter, 18, of Danville; and Dylan Lahr, 18, of New Columbia, to have their cases transferred to juvenile court. Randy Budd said he is waiting for a medical decision on the next step toward replacing a plastic skull. His wife currently does not have a forehead or parts of the right side of her head, and she wears a helmet when she is moving around. "She's happy," Randy Budd said. "I ask her every day, I go, 'Sharon, are you happy?' and she says, 'I'm happy.' I go, 'great.' She has a really great attitude."