ADAMS TWP — Mars High School reopened Tuesday monring after a bomb threat forced the evacuation of the building just after lunch Monday. District spokesman Josh Schwoebel said Tuesday that the opening came with “heightened security.” The threat came on a written note left in the girls’ locker room. Police vehicles from Adams and Middlesex townships and Mars blocked all entrances to the high school, and police briefly closed Route 228 so high school students could walk across the highway to the middle school. Superintendent William Pettigrew could be seen at 2 p.m. hurriedly leading officers from a truck marked Allegheny County Explosives Protection Team into the school’s main entrance. Soon after that, officers led a dog from car to car in the parking lot at the high school. A letter appeared almost immediately on the district’s website notifying parents of the situation, including the cancellation of all sports and activities Monday evening. Students were dismissed from the middle school, where a line of buses snaked down Three Degree Road at 2:20 p.m. Sophomore John Jackson, who walks to his grandmother’s house near the school at dismissal each day, said an announcement came over the school’s public address system after lunch that the school was being evacuated. Jackson said students were told to exit the building and walk to the middle school. He said students were not permitted to go to their cars because the school was on lockdown. He said high school students were led to the middle school gymnasium and common area, and bus numbers and locations for dismissal were announced. He said student drivers were told to take the bus home or make other arrangements, which resulted in a rush to use the phones in the middle school office. He said students at first thought the initial announcement was a drill, but began to get concerned when they saw teachers gathering up items and counting heads. “After 20 minutes, people started feeling it was serious,” Jackson said. “There was a general uneasiness because we didn’t know what was going on.” He said once inside the middle school, high school Principal Todd Kolson told the students via a megaphone that a note saying “Good luck with the bomb. Have fun Mars High” and signed only with the letter A was found stuck to a stall door in the girls’ locker room. Jackson said Kolson told students who had driven to school that an e-mail would be issued on the process for students to retrieve their vehicles.