Pupils at a Wimbledon independent school have attacked its "militant" response to an annual day of mischief which led to the school being put on "lockdown" and pupils reportedly facing expulsion.

On Tuesday, boys at King's College School took part in "muck up day", an event in which sixth formers finishing their time at the school play pranks and tricks.

Around 50 of them donned boiler suits to take part in activities including pelting each other with eggs and water bombs on the school fields, rearranging furniture in the buildings and blocking staircases with hundreds of cups of water. One pupil set off a homemade smoke bomb on the playing field.

However, the boys quickly found themselves the target of the school's new security measures, which were reportedly introduced after a break-in last month in which pupil's mobile phones were stolen.

They said a new alarm system was used to automatically shut gates and doors to all empty rooms, leaving the school in Southside "locked down" by 9.15am.

A group of boys tried to escape over fences and hedges but many were detained by the school's new security guards and porters. The pupils claim that those caught were threatened with expulsion and removal from this summer's A-level exams.

A number of students filmed the pranks but said their cameras and mobile phones were confiscated by staff and the footage deleted.

One sixth form pupil, who asked to remain anonymous, described the school's approach as "militant" and said: "They have gone overboard in their response. We were mild compared with other years."

It is understood that the boy who released the smoke bomb, which scorched part of the school rugby pitch, is facing expulsion - although he will still be allowed to sit his exams.

Another boy involved, who also requested not to be named, said that most of the school staff enjoyed the pranks. He said: "The day was immensely funny - a lot of the teachers took it in the right spirit. It was a bit strange to have the new security system used on the pupils though."

Despite numerous attempts to contact the headteacher, the school refused to comment on the story.