(CNN) A high school teacher in Massachusetts was arrested on Thursday after he admitted to leaving a bullet in the school building's stairwell to "prove to the school that they needed to get metal detectors," Southbridge Police said.

Southbridge High School teacher Alfred J. Purcell III told the staff he had discovered a live round of 9mm ammunition in a rear stairwell shortly after 8 a.m., police said. The school was immediately placed on a lockdown for about an hour, according to authorities.

Authorities reviewed the building's security footage which showed Purcell, 57, removing the ammunition from his pocket and dropping it on the floor of the stairwell, then quickly leaving the area, police said.

Purcell returned about 10 minutes later and stood over the ammunition and appeared to take a picture of it with his cell phone. He used his school-issued radio to call for help from the school administration and resource officer, authorities said.

Purcell, a resident of Woodstock, Connecticut, was charged with two counts of the unlawful possession of ammunition and two counts of carrying ammunition on school grounds. He also faces charges of disturbing a school assembly, disorderly conduct, and disturbing the peace.

The teacher's contract reportedly won't be renewed

Purcell, a biology teacher, said he did not think the school properly handled a separate incident when bullets were found in the school earlier this week, CNN affiliate WCVB reported.

Purcell told Southbridge police he wanted to "prove to the school that they needed to get metal detectors."

Police later searched Purcell's car and found 102 live rounds of ammunition for a .20-gauge shotgun in his trunk on the school grounds.

Authorities also discovered that Purcell's license to carry a firearm was in expired in Connecticut.

"This individual acted in a disgusting manner. I feel very badly for our students because it's an adult who's failed them," Southbridge Public Schools Superintendent Jeffrey Villar told reporters, according to WCVB.

"It's absolutely abhorrent that this individual decided to try to make a case," Villar said.

Prior to Thursday's incident, Villar said Purcell had been told his contract wasn't being renewed for next year, WCVB reported.

Purcell is on leave from the school, the station reported.

Purcell, a veteran teacher, was held on $500 cash bail after his arraignment Thursday at Dudley District Court, CNN affiliate WBZ reported.