A 19-year-old former high school custodian pleaded guilty yesterday to the first-degree murder of a business teacher at the school in St. Mary's County.

The custodian, Lester B. Broome, originally had pleaded not guilty by reason of insanity in the death of Beverly Jo Heater, 33, of Mechanicsville. She was raped and stabbed to death while eating lunch at her desk at the Chopticon High School last August.

In a hearing in Prince George's County Circuit Court yesterday, Broome changed his plea and also pleaded guilty to charges of first-degree sex offense and petty theft. In exchange, prosecutors agreed not to seek the death penalty and to drop other charges against Broome, including first-degree rape and armed robbery charges.

The case was moved to Prince George's County at Broome's request following heavy publicity in St. Mary's County.

Judge Jacob S. Levin set sentencing for July 9. St. Mary's assistant state's attorney Walter Dorsey said the maximum penalty Broome could receive would be consecutive life sentences for the murder and sex offense charges plus 18 months for the petty theft charge.

Chopticon High School Principal Dwight Chakales said, "I'm relieved that our community won't have to go through the ordeal of a long, drawn-out trial. All we can hope for is that justice will prevail."

Broome, a former student at Chopticon High who was working at the school under a federal jobs program, probably knew Heater, school officials said, but did not have any classes with her. Heater had taught business classes for 13 years and had been in charge of the student bookstore and youth employment program, they said. At the time of her death, she was organizing student classroom schedules for the fall semester.