Rathkeale College boys make over half of the cast of 44, and Ms Simpson said the young thespians are relishing their roles on stage.
"It's such a great show for boys - there's roles for younger boys, older boys, singers and actors."
"We have a lot of juniors [Year 9 and 10 students] involved, and their development on stage has been enormous.
"They develop at different stages as well - some of them blossom right at the end."
Ms Simpson said she came up with the idea to do Big River after her daughter appeared in a production of it at Downstage Theatre in Wellington.
"I took some students with me to see it, and they absolutely loved it.
"It's not as well known as other musicals, but it's got a great story. There's a friendship between two unlikely characters, and they go on a big journey down the river and meet all kinds of people."
Musical director and Rathkeale music teacher Adam Gordon said the musical's soundtrack is a huge part of its appeal - and has exposed the students to different types of music, such as blues, bluegrass, gospel, country and ragtime.
"[The soundtrack] is an attractive mix of easy listening styles," said Mr Gordon.
"Plus, it's a buzz for the kids to sing along with a live orchestra. We've got harmonicas, violins, banjos, clarinets and trumpets.
"It's a great combination. The songs are very catchy."
Ms Simpson said there are some stand-out characters in the show to watch out for - including Tom Sawyer's slave gang, the "colourful" chorus girls and Huck Finn himself, played by Year 10 student Callum Finn Reason.
"Callum is very sharp and astute - he works hard," she said.
"He has the job of holding the play together. So, he barely leaves the stage."
Big River will run from Friday, July 5 until Tuesday, July 10, with a matinee performance on Monday, July 8.
More information on times and ticket prices is available on http://www.rathkeale.school.nz/bigriver.