Police described both suspects as white males. One is aged in his late teens to early 20s, of a medium to large build, and about 6ft tall. He was wearing white sliders at the time of the incident.
The other suspect is described as being about 5ft 5ins tall, of slim build and wearing black clothing.
St Peter’s High School pupil Jimmy said the situation was “really scary” and he was still in shock.
After getting into the Tuffley school for the day, he said his friends came in and told him something had happened near his home.
He said the school’s alarm then went off and the computer he was on displayed a message saying “This is a lockdown”.
“The computers are not accessible at all and we’ve all had to turn the lights off, lock the doors, turn all the blinds down, and we’ve had to get on the ground,” he said.
“At the time I was scared. My heart was racing,” he added.
Jimmy said pupils were locked in their classrooms for two to three hours.
“[When police cars arrived] I thought this is getting serious, I’m getting worried, I don’t know what’s going on,” he said.
The 15-year-old said he was able to message his mother when the school let students use their phones after about an hour.
“When I was able to eventually get out, she was giving me a bunch of hugs,” he said.
“What I am is shocked. I’m surprised that this happened,” he added.
Jimmy said the school had previously run lockdown drills but a real event had never happened before.