Firefighters are investigated a suspicious device reported at Crawford High School in El Cerrito.
Officials were called shortly after 2:30 p.m. to 4191 Colts Way.
X-rays taken at the scene were inconclusive, so emergency crews detonated the device up as a precaution shortly after 4 p.m.
Bomb squad investigators were then able to confirm that the device was indeed a hand grenade.
A fire department spokesman said the grenade had been altered and hidden there, and such actions are criminal.