There was excitement in the air on the band room Sunday afternoon at Austin High School, an excitement you wouldn't expect from a group of high schoolers asked to come to school on a weekend. Giddy parents circled and poured out of the room with smartphones at the ready.

"Is he here yet?" a student asked outside the band room. She looked again out the door and paced back and forth. "I have to be to work at 5."

"You might not make it," said band director Brian Frock, who has been with the band at Austin High School for more than 30 years.

"He's coming," another student yelled. The girls ran out the door to greet the man of the hour, Marcus Mumford of British folk-rockers and Austin City Limits Music Fest 2019 headliners Mumford & Sons.

Mumford was there to rehearse with the Austin High band, a group of 100 students from all four grades. Mumford invited the band to play with him on stage Sunday night, and the band quickly assembled to to rehearse.

It all happened in a matter of hours.

During a routine phone check in earlier Sunday with fest organizers C3 Presents, Austin High School Principal Amy Taylor said Charlie Jones, one of the founders of C3 Presents, asked if the school needed anything. Taylor said she mostly jokingly mentioned the high school band would love to play with Mumford & Sons again. (The band previously played with them at South by Southwest 2011 and again in 2012.)

But Jones sprang into action. A few hours (and urgent texts and calls later) and students were rounded up. A rehearsal with Marcus Mumford was underway.

"It's just pure excitement here. We feel very honored," Taylor said. "We're so proud and thankful that C3 and Mumford and Sons support these young people. These kids have put in a lot of time and energy into this from a young age, and this sort of thing is the highlight of their band experience."

Frock said this is an event without equal for these young musicians.

"Band kids don't get to play to audiences like this. This is just going to be something that they'll never forget," he said. "It's very unique and a side of music that 99 percent of musicians don't get to see. They're going to love it."

Finally, a hush spread across the room as Mumford entered. "Hey, everybody. Sorry we had to ruin your Sunday," he joked. "The other boys send their love, by the way. They haven't quit ... I think."

Mumford then invited the band to play one of their own songs. "It's a long way to come for one song," he said. "And I'd be honored if I could play it with you, if you've got a spare drumstick."

Then he did just that. The band fired off their selection as Mumford, nodding his head with the beat, circled the band and made his way to the drumline, where he grabbed a pair of sticks and joined in.

Mumford returned to the front of the room, tuned his guitar and tried to calm the eager young players' nerves. "Last time we played this with your predecessors, we played in the wrong key. Can't get much worse than that," he said.

Then they moved on to rehearse "The Cave," one of Mumford's biggest hits being performed live in a room for no more than 200 people. Mumford played unplugged and unaccompanied, his voice filling the space. The drums joined in with a driving beat, to a sound of approval from Mumford. Then the rest of the band, slowly building from verse to verse to a rousing closing. As the song ended, Mumford grinned. "Wow. We've played with some really cool people, but I think you all might be the coolest."

Then it was on to Zilker Park. Click here to read more about that set.

ACL FEST 2019: Six days, all our coverage