TORONTO - As the daughter of musicians at the renowned Stratford Shakespeare Festival, Britta Johnson used to frolic in the orchestra pit as a child, daydreaming she'd one day be on the playbill there.

She never fathomed she'd see her name headlining a production when she was just 18.

"It's incredible," Johnson said breathlessly during a recent interview as she discussed mounting her high-school musical, "Big Box Story," at the southwestern Ontario theatre festival's historic Avon Theatre on Aug. 31.

"I mean, growing up in Stratford, that's kind of the dream, that's the stage you always see yourself on because that's the one I've always grown up with."

"Big Box Story" isn't officially a part of the Stratford festival, where her mother, Holly Shephard, still blows the trumpet and her late father, Jerry Johnson, played trombone.

But it is running for one night only on a festival stage in the middle of the festival season, giving it the kind of exposure that even veteran playwrights yearn for.

It's also the first student production being put on at the 1,090-seat Avon stage since "King Whistle!" was staged some 30 years ago.

The late playwright James Reaney wrote that show to celebrate the 100th anniversary of Stratford Central, the high school he attended and the one from which Johnson just graduated.

"I don't even think I've fully processed it yet," said Johnson who wrote and is directing the show. "It's beyond anything I ever imagined for this show."

"Big Box Story" premiered in May at Stratford Central, and sold out for its entire five-day run. An encore performance had to be added due to high demand. The idea to mount the show at the Avon was prompted by the buzz and by a letter-to-the-editor a fan wrote to a local paper.

The musical begins in HoggsMart, a retail outlet where the evil CEO, Harold Hoggs, holds a sales competition for employees. The prize is a trip to a theme park owned by a competing store. Hoggs's plan is to sabotage a roller coaster in the park so his nemesis will look bad and sales there will suffer.

"It's just a corporate world gone out of control -- a pretty heightened world but hopefully a comment on what I think about ... corporations in general," Johnson explained over the phone from her home.

"It's an exaggerated comment, certainly, it's all very silly but I thought of it because our town has been in a pretty heated fight against Wal-Mart for the past few years so it was pretty topical at the time."

"Big Box Story" is the second play Johnson has written after "AdoleSense," which won an award at the Seard Drama Festival. She started writing this latest play when she was 16, and earned two co-op credits for it.

As Johnson toiled over seven drafts, she received advice from several local theatre heavyweights, including Keira Loughran, co-ordinator of new play development at the Stratford festival, composer Leslie Arden, and Eric Coates, artistic director of the Blyth Festival.

Dancer-actress Barbara Fulton crafted most of the choreography and Paul Silton, a piano player/composer in town, helped record the scores.

"It's just been amazing how much the community stepped up to help me," said Johnson. "I feel so much gratitude. I wouldn't be here without any of them."

About 36 student cast members will be in the Aug. 31 show. Johnson will be showcasing her musical skills.

"I play piano at the side and laugh the loudest at all the jokes embarrassingly," she said.

Johnson's next artistic ventures are theatre and music studies at the University of Toronto starting in September.

As ideas for other plays swim around her head, she isn't sure she wants to be a playwright forever.

"I'm still defining myself as a young person in this world," she said.

"In the past little while I've certainly taken on the role of a writer and director more than a performer but I love to be on stage too. I hope to do all of it in my life."