Police found a man suffering an “injury to [his] abdomen” and he was taken to hospital.
His injuries are not thought to be life-threatening.
A man in his forties has been arrested on suspicion of grievous bodily harm (GBH).
In a statement to parents, a spokesperson from Goodmayes Primary said: “Unfortunately today at around 3.45pm there was a serious incident that involved a member of the public in the vicinity of the school.
“The police and emergency services dealt with this incident promptly.
“Staff at Goodmayes Primary School supported the police and ambulance service and we ensured that pupils were dismissed away from the police cordon."
A Met Police spokesperson said: “A 43-year-old man was arrested on suspicion of grievous bodily harm.
“He remains in police custody at this time.
“A crime scene remains in place in the area.”