MILFORD – Three teen-agers are under house arrest after being charged in connection with four bomb threats in five weeks at the Lewis S. Libby School.

The two 14-year-old girls and a 13-year-old boy were arrested Tuesday evening and charged with multiple counts of terrorizing, a Class C felony, according to law enforcement officials. The eighth-grade girls and seventh-grade boy have been suspended from school and face possible expulsion.

The school was evacuated Tuesday morning when a note was left in a hallway indicating the school would be blown up, Deputy Bill Flagg of the Penobscot County Sheriff’s Department said Wednesday.

The first threat, made on Valentine’s Day, also was written and left in a hallway. The other two threats were written on a bathroom wall, he said.

“The threat level and intensity increased,” Flagg said Wednesday. “With each subsequent incident, the threats were more to the point and of a more serious nature.”

He said the bomb scares apparently began when the girls dared the boy to write a threat so students could get out of classes. Each time a threat was discovered, the school was evacuated and searched, and classes were cancelled for one or two hours.

Keith Ober, Union 90 superintendent, said Wednesday that children in the lower grades at the kindergarten-through-grade-eight school expressed relief when they heard about the arrests. He said they had been frightened by the continuing threats.

“These kids [who made the bomb threats] did something to explore their boundaries, which is what seventh- and eighth-graders do,” he said. “But this was unacceptable.

“We will make a strong request to the court for restitution and encourage the law to go all the way and hand down significant sentences,” the superintendent said.

Two years ago, three 14-year-old boys were arrested for making bomb threats at the same school after a “suspicious device” was found in a classroom. The harmless device was fashioned to look like a pipe bomb, according to police.

A meeting with parents and staff to discuss Wednesday’s arrests is scheduled for 6 p.m. today. A school committee meeting is scheduled to start at 7 p.m.

The trio charged with making the threats is expected to appear May 22 in 3rd District Court in Bangor.