Pupils at the Roman Catholic John Henry Newman secondary school in Stevenage were shocked to find the Duke of Cambridge sitting among them eating lunch.
The heir to the throne was on a shift in his role as a helicopter pilot for the East Anglian Air Ambulance and the team needed to refuel on their break.
Students discreetly took pictures of the Prince as he ate - it's hardly likely their parents were going to believe them otherwise - and uploaded them to Twitter.
Headteacher Clive Matthew, said; 'The school is close to the Lister hospital so the helicopters use the playing field to land and we always offer the crew refreshments.
'We were stunned when we realised Prince William was part of the crew today... he sat with me and some year 11 pupils and they asked him if he had enjoyed the Christmas holidays and he told the students he'd had a lovely time.'
Look who Joe had lunch with todayâ¦ Prince William! pic.twitter.com/EOzBRxYs0p— lisa kasp (@lisakas) January 13, 2016
The 33-year-old father of two might be used to enjoying Royal banquets at Buckingham Palace, but it seems he was satisfied with the offerings and opted for a plate of roast chicken and cauliflower cheese.
In the snaps, William is seen wearing his uniform of black overalls and his glasses, which he is required to wear whilst flying.
One student was particularly disappointed to have missed school on Wednesday, although she wasn't sure whether she would rather have the Prince or cannelloni for lunch.
And as every good guest should, at the end of lunch William paid his compliments to the chef, Stuart.
Formula One driver Lewis Hamilton and England footballer Ashley Young both attended the school in Hertfordshire.