If Austin City Limits Music Festival has never seen a bigger crowd than Lizzo, it’s likely never seen a more packed stage than the one for Mumford & Sons. The British folk act closed out the last night of the fest for 2019 with a surprise addition to their Weekend One set: the Austin High School marching band.

To cheers from the crowd, the Austin High band marched onto the stage, drumline playing, about halfway through Mumford & Sons' two-hour set. The players kept piling on until they practically consumed the massive headliner stage, joining the band on “The Cave."

“There are about 100 kids up here. … Austin, this is your future right here,” frontman Marcus Mumford said. “At 11 a.m., none of us knew this was going to happen. That's the can-do spirit of Austin High, which I think speaks to this city, and it's incredible."

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The surprise addition was the result of some quick collaboration between Austin High School and C3 Presents. The marching band elevated the song to triumphant heights, building slowly until the finish, with Sousaphones swaying and the crowd going wild.

Mumford & Sons previously headlined ACL Fest 2016, and Sunday night’s expansive crowd proved they're still a big draw for festival-goers. Mumford said he was humbled to be back.

"This is one of the greatest honors of our life," he said. “This is a lot of people. It’s really a sight to see.”