It was close to 2.15pm when a group of around 20 youths armed with weapons including guns and knives invaded the Seomara da Costa Primo school in Amadora, looking to wage revenge on a 17-year-old student.
A 58-year-old school gatekeeper was attacked after being hit on the head with the butt of a gun as he tried to stop the gang from entering the school grounds.
A 50-year-old teacher was also assaulted in the intrusion.
The gang managed to reach their victim and kicked and beat him before fleeing the scene.
At the time of going to press the assailants remained at large. Many of them are believed to attend a school in Cacém.
This latest incident comes two years after a similar occurrence at the same school, when a 19-year-old student was stabbed outside the school and died inside its grounds, in another attack relating to rival gangs from Sintra.