The Chainsmokers had a show in Chicago on Saturday night, and I guess they decided they needed a little warm-up.
They crashed Huntley High Schools' prom, which happened to be going on at the Hyatt Regency (right across the street from the All State Arena). Their appearance actually WAS pre-planned though; the principal of the school said it was one of the hardest secrets he ever had to keep.
(Thumbnail: Getty Images)