"There has been a shooting at Kingdom School in Riyadh," the US mission said on its Twitter account, warning that the "shooter (was) still at school".
"School (is) closed, no children present. Please avoid area," the Twitter post said, without elaborating on casualties or motives behind the shooting.
Most schools in Saudi Arabia are on holiday.
Opened in 2000, the school is owned and operated by Kingdom Holdings, which belongs to Saudi billionaire Prince Alwaleed bin Talal.