"We have some heroic and brave faculty members," Schools Superintendent Dallas Dance said. "They responded very quickly to minimize damage."
Seth Warner, a youth pastor at the Faith Fellowship Church across from the high school who graduated in 1999 with Wasmer, said he was not surprised to hear that the guidance counselor had intervened. He described Wasmer as "not big, but built."
"I knew that if anyone could take him down, it would be Jesse," Warner said.
The school was evacuated, and students were escorted to a nearby shopping center and middle school.
Perry Hall is a middle-class community along the Interstate 95 corridor, northeast of Baltimore city. The school is the largest in the county, with 2,200 students.
County Councilman David Marks, who lives next door to the school, said he had received dozens of phone calls and text messages from worried parents and residents.
"This is a very comfortable, very safe community, and it's an excellent high school," said Marks, who graduated from Perry Hall. "I think this is an aberration, but clearly one that is horrifying, particularly on the first day of school."
Police planned to provide additional security when the school reopens on Tuesday, and stress counselors were called in to work with students, faculty and staff.
Television coverage showed scores of police cars surrounding the school and parked on neighborhood streets. A group of officers with weapons drawn staked out a corner of the building, one of them lying prone on the ground and appearing to cover a particular area of the campus. Hundreds of students streamed away from the school.
Cathy Le, 15, said students were panicking as they tried to find out what was happening. They texted and called each other frantically as they were locked in their classrooms for more than an hour, she said.
At the scene, buses, emergency vehicles and parents in cars filled the roadway between the high school and the shopping center. There were obvious signs of relief displayed as parents found their children.
Kristin Kraus, whose son James attends the school, described hearing about the shooting as "absolute terror." However, Kraus said, "within a couple of minutes he texted my husband that he was OK."