The unnamed boy, 17, believed to be from London, was pulled from the water at a spot popular with celebrities off the Sussex coast.
It is believed he was one of around 50 pupils on a coach trip to West Wittering on Tuesday morning.
Emergency services scrambled after reports of a child in difficulty at around 1pm on Tuesday.
Other children, believed to be his classmates, were visibly distressed on the beach after he was airlifted to hospital.
A spokesman for Sussex Police said: “Emergency services and HM Coastguard were called to concerns for a boy’s welfare in the sea off West Wittering beach at around 1pm on Tuesday, July 2.
“A 17-year-old boy was airlifted to hospital, where he was sadly confirmed deceased.
“His next of kin have been informed.
“An investigation is underway to understand the full circumstances of the incident.”
Met cops broke the news to his parents on Tuesday afternoon.
An ambulance spokesman confirmed the boy was flown directly to hospital by the Coast Guard.
An eyewitness told of “horrendous” scenes as the tragedy on Tuesday unfolded.
Speaking to The Argus, the witness said: “I was at the beach from around 1pm. Suddenly there were loads of police and coastguard.
“I could see lots of upset children and people from what looked like a school trip.
“It was absolutely horrendous. The tide was out at that time.”
The rolling sand dunes and wide beaches at West Wittering are popular year-round.
The area is also known for its links to celebrities.
It’s home to superstar actress Kate Winslet, and locals have previously included names like Brit actor and singer Michael Ball, actor Nicholas Lyndhurst and Rolling Stones front-man Keith Richards.
According to the local tourist board, a million beach-goers flock to the area each year.
Up to 20,000 people hit West Wittering Beach on single days during peak summer, its website says.
Due to its popularity, there are many instances of swimmers getting into trouble at sea.
The website adds that 190 people require lifeguard assistance in the water via rescue board or jet ski rescue every year.
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