Police were called to a school reunion after reports of a group of middle-aged rebels vandalising the building, smoking in toilets and deliberately setting off fire alarms.
Hundreds of adults - aged from their late teens to 70s - attended the evening at the old Orwell High School in Felixstowe on Monday.
But the event had to be cancelled as police and firefighters were called after reports that a group, said to be in their 40s and 50s, were misbehaving.
Those who attended took to Twitter to describe lewd pictures being drawn on walls and alcohol being smuggled in.
A Suffolk Police spokeswoman said officers were initially called at about 7pm to reports of parking problems but were then alerted to the trouble and the building had to be evacuated.
She added: "The fire alarm had been triggered and the building had to be evacuated.
"Concerns were highlighted to us about vandalism and writing on walls.
"We are investigating and may consider prosecutions for criminal damage."
The building is due to be demolished as part of a £20 million Felixstowe Academy project and the event was designed to give former pupils one last chance to relive their school days.
In a statement, the school said the vast majority of those who attended had behaved themselves.
It added: "Overwhelmingly the response to the event was positive, with visitors representing over 60 years of association of the three legacy schools.
"Unfortunately the evening had to be brought to a close earlier than planned due to health and safety concerns resulting from the setting off of fire alarms.
"Sadly a very small minority of adult visitors took it upon themselves to smoke within the building and write graffiti on the walls, causing vandalism to an operating school."
A similar event which was due to take place at another building last night was cancelled.