A bloodstained T-shirt in a dustbin. Two getaway cars by a lock-up garage. A gang of young men in smart suits brandishing shooters and brazenly videoing themselves in an east London alleyway. To one alarmed pensioner and 30 armed police, this terrifying scene in Barkingside was another example of the spread of youth gun crime.

But when members of SO19, Scotland Yard's firearms unit, swooped on the gangsters after an hour of surveillance, they found eight petrified A-level students filming their Media Studies coursework with three toy guns bought from a sweet shop.

Now the pupils of King Solomon high school, Barkingside, are worried they will fail their A-levels after officers seized their video coursework.

"It was absolutely shocking," said Adam Langleben, 18, one of three students who wrote the script for a 15-minute film about gangsters and persuaded five friends to star in it. "Round the corner I heard people shouting 'get down, get down' and 'against the wall'. I thought my friends were joking and then I turned and saw a gun pointing in my face. We were lucky not to be shot."

Mr Langleben said they had not checked with the police because they were unaware they needed permission. After discarding a T-shirt daubed with fake blood, they were just finishing at 2pm when the SO19 officers pounced.

Searched twice, Mr Langleben claimed that most of the elite unit "were having a laugh" when they realised it was a false alarm. "But one guy went crazy at us. They ticked us off at the scene and told my friend, the director, if he didn't sign the tape over he would be arrested."

The students must report to the police station at 7.30am today to view the video with officers.

Mr Langleben, who needs an A grade in his Media Studies course to take up his place at Leeds Metropolitan University, said he was worried about missing the coursework deadline of mid-June and failing the A-level.

A Metropolitan police spokesman said the video would only be returned when they had completed their investigation and they were happy that no crime had been committed.