The other pupils were very disappointed with their peers - they were shocked and they did feel let down

Moira Humphreys, head teacher

Two of the boys, both 15, were arrested on suspicion of possessing a substance believed to be cannabis.

The other boy and girl, also 15, were arrested on suspicion of possessing the drug with intent to supply, a spokesman for the Metropolitan police said.

The school's head teacher Moira Humphreys said the other 150 Year 10 pupils on the trip felt let down by the four.

'Disappointed'

"They were very disappointed with their peers - they were shocked and they did feel let down."

Ms Humphreys said she was not able to comment on individual cases, but confirmed the pupils had been disciplined.

"We have carried out a full and thorough investigation and have dealt with the situation accordingly," she said.

A total of 700 pupils from across the school had been involved in trips this week and had behaved impeccably, she added.

A spokeswoman for Chessington World of Adventures said the park carried out random searches of visitors who were behaving suspiciously.

"Our security team found something on the children that we considered suspicious and called the police," she said.

One of the boys had accepted a caution and the other three pupils were bailed to return to Kingston police station on 14 August, police said.