The first expansion of the Bridgehampton School District’s school building since it was built in 1930 is being proposed to bring the facility into the 21st century, said Superintendent Lois Favre.

There is such a lack of space to accommodate a growing enrollment and other needs in the two-story brick building — which teaches 209 students in kindergarten through 12th grade — that four portable trailers are used for offices and to conduct classes. The first was placed there about 50 years ago.

The space crunch has also resulted in the library housing some classes, elementary art and music lessons being conducted in the school’s café and the basement being used for students’ locker rooms and two technology rooms.

“At one point every classroom on our second floor was split in half to handle the number of classes we need to offer; now they are too small to house our larger class sizes,” Favre said. “Most classes are multi-use and are shared by more than one teacher.”

Enrollment at the school on Montauk Highway, once attended by the Boston Red Sox’s Carl Yastrzemski, is up 35 percent in the past three years, Favre said.

Robert Hauser, assistant superintendent for finance and facilities, said the expansion will cost approximately $25 million. If the project is approved by the school board, a special referendum would be held Dec. 12, Hauser said, adding that construction could start in July 2018 and take about 13 months to complete.

He attributed the enrollment increase to the June closing of the Wainscott-based Child Development Center of the Hamptons and interest from students at private schools. Hauser said that the latter have been attracted to some of the school’s newer offerings, such as digital photography, 3-D printing, a botany program and lunches prepared by school cooks instead of those provided by a vendor.

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“The discussion [about an expansion] has been noted in board minutes since 2005,” Favre said. “Creating a 21st-century learning environment in a building built in the 1930s is challenging at best. And add to that our increased enrollment, we have reached a point where there is no other option but to add instructional space. Safety and the need for appropriate instructional space are the driving factors.”

An official push came in the spring when the Middle States Association of Colleges and Schools recommended a facilities upgrade.

Hauser said that though plans for the expansion are still in the initial phase, drawings have been done by Port Jefferson-based architect John A. Grillo.

“We’re at the stage right now where we’re getting input from the staff, community and students,” Grillo said.

A first community forum on the expansion proposal was held in September, and another is scheduled for Oct. 15 from 9 to 11 a.m. in the school gymnasium.