The Chainsmokers Crash Huntley High School's Prom

The Chainsmokers gave a class of seniors in Chicago a gigantic surprise when the boyz crashed their prom.

The DJ'ing duo played a surprise set Saturday night for Chi-Town's Huntley High School at at the Hyatt Regency hotel, right after their concert across the street at the Allstate Arena.

Turns out the performance was pre-planned -- and free if you can believe it. Some kid from the school had contacted their management a couple weeks before to schedule the "impromptu" show. Most of the kids at the prom had no idea they would show.