Capt. Martin Flatley initially said multiple shots may have come from a BB gun fired outside the school, shattering a classroom window.
Later, a police press release made no mention of a BB gun, only saying that a "small caliber round" was recovered inside the classroom.
An 18-year-old girl was struck in the head by an "unknown projectile," according to the police statement.
Several media outlets identified the student as senior Amanda Olsen. The girl's parents told Fox 5 News that their daughter could "see the bullet" coming at her.
At least one other student suffered minor injuries resulting from shattered glass, police said.
A suspect, whom the school identified as a non-student, was arrested, according to a message from superintendent James McKenna.
"Everyone is safe," Mr. McKenna said in his telephone message to parents. "I repeat — everyone is safe."
Capt. Flatley added: "We feel we have the person who fired the gun."
A police press release declined to name the suspect, identifying him only as a 28-year-old male.
The school was on lockdown for nearly an hour, beginning at 1:53 p.m., while police searched the grounds. Dismissal time was scheduled for 2:09 p.m.
Parents waited in the parking lot, wondering aloud when their children would be released.
Students exhibited relief after being released from the school.
"The principal gave us some sort of warning and then all of a sudden we were in our lockdown drill," said senior Tabitha Wells. "We all sat against the wall and had to remain silent."
After-school activities, including sports, were canceled for Thursday.
Elementary school dismissal was also delayed by an hour as the district worked on safely transporting the high school students home, Mr. McKenna said.