Locker rooms at St. Croix Preparatory Academy are so tiny that when athletic teams squeeze in to change into their uniforms, it's akin to the 1950s fad of college students stuffing phone booths.

Meanwhile, the adjoining gymnasium has to double as a concert hall, but it is booked solidly with games and practices, and band and orchestra students have to compete with athletes and other school events to win a night to perform.

But not for much longer. Officials at the rapidly growing charter school in Baytown Township are about to embark on a $5 million expansion that will add 23,000 square feet to the building, including expanded locker rooms with showers, a fitness and weight room, an expanded cafeteria, six classrooms and a large multipurpose performance space.

"It will allow us to flush out the three A's -- arts, academics and athletics" -- which are at the core of the school's mission statement of developing academic potential, personal character and leadership skills," said Executive Director Jon Gutierrez. "We've grown past capacity, and this will allow us to accommodate our growth."

With approval from the Minnesota Department of Education, construction is set to begin later this month and will be finished by March. The school's authorizer, Friends of St. Croix Preparatory Academy, will issued bonds to foot the bill. No public money will be used, Gutierrez said.

Along with the extra space, the school, which serves students in kindergarten through 12th grade, also is adding a handful of new teachers this fall to keep up with increasing enrollment. There will be 70 teachers on staff.

Founded in 2004, St. Croix Prep opened with 200 kindergarten through 7th-grade students in a small space on Myrtle Avenue in downtown Stillwater. The school added a grade each year, and relocated to a space next to Trinity Lutheran Church in Stillwater and later to an empty strip mall in Oak Park Heights. In 2009, when it had all 13 grades and had its first senior class, St. Croix Prep opened a $19 million facility designed for 1,000 students on a 60-acre site on Stagecoach Trail.

With more than 1,100 students enrolled this year and up to 1,200 expected in 2013-14, the expansion is needed to alleviate logistical and space problems, Guiterrez said.

"It will allow for more efficient use of the space," he said. With showers, "it will allow for practices before school. When practices [or games] went later into the evening, that was problematic when other events were going on in the building."

In recent years, St. Croix Prep has expanded its high school athletics program to 15 varsity sports, and it won the state archery championship in 2011 and 2012.

But sometimes teams had to be bused off campus for events because the gym was needed for band and orchestra concerts, school assemblies, elementary school book fairs and all-school fundraisers. Now those events will have a place of their own in a 7,000-square-foot auditorium, Gutierrez said.

In conjunction with the expansion, the school was reconfigured over the summer. Most of the top floor of the two-story building will used for the high school. A section of the second floor on the building's west side will be used for some fifth- and sixth-grade classes, and the lower floor will be used for elementary grades and middle school.

The new space will include a fourth science room and lab and give the school a total of 52 classrooms. That will allow the school to continue offering a wide range of courses -- such as those in printmaking, pottery and ceramics -- and keep "our commitment to the arts and increasing our athletic programs," Gutierrez said.

Tim Harlow • 651-925-5039 Twitter: @timstrib