The 17-year-old suffered serious head injuries during a ‘fight’ which took place between 2pm and 3pm in the area of Whitely Close and Sutton Court in Grands Vaux, according to the States police.
It is understood that both of those involved are known to each other.
A 15-year-old boy has been arrested on suspicion of grave and criminal assault and remains in police custody.
In a statement, the States police said: ‘The police urgently want to speak to anyone who saw the incident or who may have information that could assist with this enquiry.
‘Anyone with information is asked to contact the Serious Crime Unit on 612200 or the independent charity Crimestoppers anonymously on 0800 555 111 or via their anonymous online form at Crimestoppers-uk.org.’