Superintendent Curtis Griffin said the lockdown, which went into effect around 7:40 a.m. Friday had been lifted by about 8:25 a.m.
"We were asked to go into lockdown by Montgomery County and police due to the police activity in the area," Griffin said. "I'm not sure what they were doing."
Children arriving to Simmons during the lockdown were rerouted to Keith Valley Middle School, Griffin said.
During a school lockdown, no one is permitted to enter or exit district schools.
The school day has since "returned to normal," according to Griffin.
Horsham Police did not return calls regarding the incident. Griffin said Friday afternoon that he did not have specifics about what prompted the lockdown.
District officials have reviewed what went well and what needs improvement in terms of lockdown procedures, he said.
Paramount is to "provide parents with more effective information," Griffin said. Beyond just email notification of a lockdown, Griffin said he would like to notify parents about whether incidents are police-related or district-initiated.
"We're going to work through that," Griffin said.
The message that was sent out Friday morning was "pretty generic," he said, adding that the goal is to "communicate quickly and also effectively."