Katie Mayhew sang twice at Symphony Hall in Boston yesterday, first in rehearsal and second as part of a competition for the Boston Pops High School Sing-Off.

Katie, 15, is one of 22 contestants in a statewide competition to be a singer with the Boston Pops this summer. They came from all over the state, from as far east as the Vineyard and as far west as Stockbridge. Each singer, aged from 15 to 18, was aspiring to be a winner. The grand winner will perform with conductor Keith Lockhart and the Boston Pops as part of the Fourth of July concert and fireworks.

After yesterday’s rehearsal performance, Katie drew applause from the other contestants and their parents. “I decided I would just go up there and have fun. For the first time I wasn’t nervous,” she said moments after singing her piece, Being Alive.

Katie was alive. Her voice, like her bare left hand, reached out.

Her melodic voice sent notes and words out across the hall and could be heard clearly out in the hallways of the old building. She made history yesterday just by singing a song, just by coming from her home in West Tisbury to sing a song she’d learned to audition not more than a month ago.

In the hall, built in 1900, gold trim and five chandeliers hung high above her.

Pianist Peter Mansfield gave Katie the go-ahead and it was her moment. The sophomore was dressed in a yellow flora tunic and leggings.

She was the 12th singer of the 22 that performed yesterday afternoon. The huge hall was mostly empty, except for the parents and support staff.

Deborah Mayhew, the singer’s mother, and her partner Todd Follansbee sat on metal chairs midway in the hall and listened intently to Katie sing the song again, as they had heard her before. But this afternoon it was all different.

When she had finished her song, all of the young high school students had a moment to meet and listen to Mr. Lockhart. They posed for a group photograph on the stage. Mr. Lockhart praised the singers. He told them that as contestants they were all very talented. He wished them luck.

Mr. Lockhart met with Katie outside the big hall, on a stairwell, after she had performed. The two talked about the Vineyard and the Pops. Katie said she had attended the Pops concerts at the Tabernacle and at Ocean Park in Oak Bluffs. The Pops will be coming back to the Vineyard again this summer.

Mr. Lockhart said the Boston Pops is always looking for talent, but this was a new opportunity to open the hall to aspiring young high school students. This was not a classical version of American Idol, he said, for the high school students are not aspiring professionals. “This is a wonderful time in their lives when they haven’t yet become professionals. This is a great moment for them and I hope it all makes a difference,” Mr. Lockhart said.

Last night, the singers performed in the hall for friends, family and members of the public.

From that concert, six semi-finalists will be selected. Those student will be announced formally Wednesday.

The semi-finalists will perform in Pops concerts on June 17 and 18, led by Keith Lockhart. The six students will be divided in half, three of them performing on June 17 and three performing on June 18.

Three finalists will be chosen to perform with the Boston Pops at Symphony Hall on July 1. The winner of the Sing-Off will perform with the Boston Pops at the Charles River Esplanade on July 3 and 4, an event that includes a fireworks display.

Katie said she had come up with the tune only a few days before the May 9 deadline. Dan Murphy, the high school and Minnesinger choral director, had gone over the piece with her. They’d recorded it on a DVD.

Yesterday afternoon Katie said she looked forward to the evening concert. “You got to think positive,” she said. “I am just going to do it and think everything is going to be okay.”