A tip for British schools planning to hold a US-style graduation ceremony: introduce a dress code so no one turns up dressed as a 6ft penis.

That was the costume chosen by Calvin Morett, a 19-year-old who gatecrashed the graduation ceremony at Saratoga Springs High School in New York state.

Morett, a former pupil at the school, donned the inflatable costume and sprayed the 5,000-strong audience with Silly String. He was arrested by police sergeant Sean Briscoe after running off stage towards the exit.

“Once I stopped laughing, he was pretty easy to catch because he was tripping on the lower portion of the costume,” Briscoe said.

He told the local Times Union that Morett’s motive for donning the phallic outfit was that he “thought it was pretty funny”.

Morett later pleaded guilty to disorderly conduct and was ordered to pay court fees and perform 24 hours’ community service. He also had to write a letter apologising to the city for his behaviour.

CBS TV news reported that Morett “did not wear a penis costume to his own graduation”.