The head teacher of The Discovery School, Newcastle, has apologised after the teenager was left behind in the capital as the rest of the group headed back to Newcastle on the train.
The pupil immediately alerted station staff at King’s Cross and was put on the next service north, accompanied by train staff in First Class, and was met by a teacher in York.
The school has said there was “no excuse” for the serious error and that it will change its policies to make sure it does not happen again.
According to reports, four teachers were accompanying the pupils on the field trip to London.
Principal Gareth Rowe said: “He was never in any danger as he had the presence of mind to immediately let station personnel know what had happened.
“He was put on the next train and looked after by the guards in first class until he could be reunited with his teacher in York.
“However, there is no excuse for what was a serious error on our behalf.
“Our first priority is always the safety of our pupils and obviously this incident highlights a failure in our processes which will now be reviewed and amended so this can never ever happen again.”