A bomb threat disrupted classes for Dartmouth High School students who were put on lock down while state and local police searched for explosives Tuesday morning

Dartmouth High School received an automated phone call, known as a robo-call, at 10:19 a.m. that claimed a bomb was in the building. After conducting a sweep with K-9 units, authorities deemed the building safe at 12:10 p.m.

School officials notified parents via phone soon after the threat was received.

Approximately 20 parents waited anxiously outside the high school’s main entrance hoping to dismiss their children while the sweep occurred.

Some expressed frustration over the school’s response.

One father named Noah, who declined to give a last name, said a similar incident occurred at Quinn Elementary School two years ago. In that case, Noah's younger son was evacuated to the middle school, but he wasn’t notified until later. At that time, he said authorities made him wait while he tried to dismiss his son.

With his older son attending the high school, Noah said keeping students inside was troubling in light of the threat.

“I’m concerned,” he said. “Why lock them in there?”

Lucia Teixeira texted back and forth with her daughter, a sophomore, during the lock down.

“She wants me to get her out,” Teixeira said, adding that students were aware they weren't participating in a drill.

Teixeira said she understood officials were following protocol by keeping students inside. However, she wondered if perhaps the policy should be reconsidered.

“In there they’re like sitting ducks,” Teixeira said. “It’s not something you feel comfortable with.”

Principal John Gould addressed the parents gathered out front twice during the situation.

He explained that officials had deemed the threat low risk. According to school policy, low risk threats are handled by keeping students sheltered in place. During a high to moderate risk situation students would have been evacuated, he said.

Gould said that students couldn’t be dismissed while the sweep occurred because it would have interfered with the search work performed by the K-9 unit.

“We don’t like the climate of the school being compromised, but safety first and that’s why we approached it the way we did,” Gould said.

He noted that an investigation determining where the call originated will be conducted.

An unidentified officer with the state police bomb squad also spoke to parents, offering reassurance after authorities searched the school. He said administrators exercised an “over abundance of caution” during the event.

“They did everything to make sure the school was safe,” said the officer.

He told parents that the situation was not taken lightly.

“You have to, at some point, trust the response. We are absolutely all over this,” he said. “The staff of this school was all over this.”

After receiving the all clear one mother, who was wrapped in a blanket against the cold, ran inside the front doors near tears.

The high school incident came after similar warnings of violence were made using robo-calls this winter.

In January, 10 Massachusetts schools received bomb threats on the same day. Most of them were evacuated. No explosives were found.

Dartmouth High School wasn't the only school that received a threat on Tuesday. Ashland High School, located near Framingham, received a robo-call warning of a bomb scare at 10:30 a.m. That school was evacuated as police searched the building. Ashland students were sent back to classes at 11:50 a.m.