MOSCOW (Sputnik) —It is yet unclear what caused the explosion, according to Aftonbladet.
"We have received a report about a loud explosion and smoke has been seen," local emergency response team head Ola Akesson was quoted as saying by the newspaper.
"Despite the fact that I was many meters away, the entire building shook. Suddenly, one of the janitors came running in yelling that there was a bomb in the sports hall. When we ran out, the emergency services were already there," Christine Holmberg, the school principle, told the newspaper.
All students have been evacuated from the area, she said. No traces of explosives have yet been found.
Karlstad is located around 300 km (185 miles) west of Stockholm and serves as the capital of Sweden's Varmland region.