Police locked down the area after they were told someone had entered the school premises with a weapon.
Students were told to stay in their classrooms and barricade the doors of Friedrich Ebert School as cops combed the area.
Police said a man who was seen with a firearm at a high school in southwestern Germany on Monday was on the loose, but that students were allowed to leave the school after a search there turned up nothing.
Police had earlier detained another suspect but released him after determining they had the wrong man.
A manhunt is still underway to find the suspect who is described as being in his teens with a dark complexion and beard.
He was wearing a red T-shirt, blue jeans and reading glasses.
Police spokesman Bjoern Reusch said the suspect didn't use his weapon and nobody was hurt when he briefly showed up inside the school building in Esslingen, near Stuttgart.
Parents were told not go to the school, with cops saying pupils would be escorted out by officers once the all clear was given, Bild reports.