Maryland School Shooter Assembled Shotgun in School Before Attack

VIDEO: Perry Hall High School student, 17, in critical condition after getting shot on the first day of
WATCH Maryland School Shooting: 15-Year-Old Suspect in Custody

A 15-year-old student brought a disassembled shotgun to school on the first day of class today, put the weapon together on campus and entered the cafeteria where he shot and critically wounded another student.

A teacher lunged at the student to stop the shooting and before pinning the boy against a wall a second round was discharged, police said.

The school was put on lockdown and then evacuated.

It was a bloody and traumatic start to the school year at Perry Hall High School in the Baltimore suburb of White Marsh, Md.

Baltimore County Police Chief Jim Johnson did not identify the suspect, but said he was 15.

The victim, who also was not identified, was 17. He was medevaced to Baltimore Regional Hospital Shock Trauma where he is listed in critical condition.

The alleged shotgun was disassembled when it was brought to school and assembled on campus, two sources familiar with the investigation confirmed to ABC News. The weapon had been concealed, Johnson said.

The gunman walked into the school cafeteria at 10:45 a.m.

"We believe that the suspect fired one round and just struck this individual. We do not believe at this time that he was targeted," Johnson said during a news conference.

A teacher grabbed the boy with the gun and wrestled him against a wall, holding the gun away from the shooter, but during the struggle a second round was fired, Johnson said.

A second staffer, a school resource officer, was outside the cafeteria and rushed in to help the teacher subdue the suspect until cops arrived, police said.

"There was an individual in particular that really stepped in to make a situation less severe," Superintendent Dallas Dance said during a press conference today. He did not identify the teacher.

A police statement later said the 15-year-old in custody was cooperating with police, who were trying to determine whether to charge him as a juvenile or an adult.

Additional police would be assigned to school Tuesday when classes resume, police said.

Students were held in their classroom for two hours before being evacuated, students told ABC News.

"I was in class in the basement of the school so we didn't hear or see anything," a student, who identified herself as Erisa, told ABC News. "But once the assistant principal, Mr. Arnold, went on the speakers and yelled, 'Lockdown' very loudly I knew it wasn't a joke."

"After we had left the classroom, we saw police and the SWAT team all around the school," student Linzee Hobgood told ABC News. "When we walked outside, there were police cars everywhere. I didn't find out what was going on until my mom called me and told me what had happened. It was shocking that there was a shooting in our school."

Students were evacuated to the Perry Hall Shopping Center where frantic parents went to be reunited with their children there, according to police. Students are also being bused home from the middle school across the street, ABC News affiliate WMAR reported.

Several other students suffered minor, non-shooting injuries during the incident.