Police were called to St Pauls Academy, in Abbey Wood, on April 1 after hearing claims of a group in masks hanging around. Officers searched the men and found one to have a baseball bat concealed in his clothing.
The man was arrested by Sergeant Geoff Warren on suspicion of possessing an offensive weapon.
The suspect in question was already on bail for grievous bodily harm and was later charged and remanded.
School staff had alerted police after checking the nearby area on the final day of term, the local force said in a statement.
A spokesperson for Greenwich Police said: “Greenwich school officers were on the last day of term patrols and received a call near St Pauls Academy of a gang of males in balaclavas waiting for pupils to come out.
“They were first on scene, got the group searched and one had a baseball bat hidden in his clothing.
“Police Sergeant Geoff Warren found the bat and made the arrest.
“The suspect was on bail for grievous bodily harm so was charged and remanded.”
The Roman Catholic academy on Finchdale Road, which counts rapper Tinie Tempah among its alumni, has around 1,200 students aged eleven to 16.