Police officers said the boy had been on Primrose Hill in north London, near the entrance to Regents Park, with four female friends when they were attacked by four males, who were all said to be white.
Detective Sergeant Steve Masterson said: “The victim in this case sustained a significant injury that continues to be classified as ‘serious’ and will require surgery.
“It is vital that we trace and apprehend those responsible for this motiveless and cowardly attack and I appeal to anyone who was in the park at around 9.30pm who saw a group of males to contact police.
“Your information, no matter how unimportant it may seem, could provide a piece of the puzzle that leads us to the suspects.”
A statement from JCoSS said: "A year 12 JCoSS student has been the target of a serious incident on Primrose Hill on Tuesday evening and is recovering in hospital.
"Students in his year group had an emergency assembly this morning to inform them and were offered further support should they need it.
"Our thoughts are with the student and his family and we wish him a speedy recovery."
Officers closed roads in the area to conduct investigations into the incident. No arrests have been made so far.
Police are appealing for witnesses to contact Central North Command Unit by dialling 101, via Twitter @MetCC or anonymously at Crimestoppers on 0800 555 111.
