Police say a Montgomery County high school was briefly locked down after school security mistook a student's umbrella for a gun.

Authorities say Upper Dublin High School was locked down Monday morning after four male students were spotted on a school security video.

School officials identified one of the students and police quickly located all four students, who were being interviewed. Police say they were working on a project illustrating the immune system protecting the body.

Monday marks the first full day of classes for most students since 20 children were killed in a shooting at Connecticut elementary school.

An Upper Dublin student said he and others awaited the all-clear in a locker room. The sophomore says it makes sense to be alert given Friday's shooting in Newtown, Conn.