School leaders were weighing up whether the cohort would be in classes at the co-educational Brisbane Street campus or in single-sex classes at the William Cowper campus.

Principal David Smith said today those future year 10 students would learn on the Brisbane Street campus, mainly in single-sex classes but with some flexibility for elective units.

It comes after the news of a master plan for beyond the school's 100^th anniversary, which it marked in February.

That includes a $4.6 million building project at the William Cowper Campus.

Work should start soon on a multi-use block there, with a performing arts area, design and technology classrooms, four science labs, general classrooms and amenities.

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The school also has plans for a different "diamond" model, in which boys and girls are educated together or apart at different times in their school life.

All primary students will be educated at the William Cowper campus in co-educational classes, as they are now; but, from 2021, years 7 to 9 will stay on there in single-sex classes.

Students in years 10-12 will have classes at the Brisbane Street campus; years 11 and 12 will be in all mixed-sex classes.

If need be, year 9 and 10 students could be in co-ed classes for their elective units.

"Calrossy is committed to enhancing and implementing our unique 'diamond model'," Mr Smith said.

"Supporting the needs of boys and girls at every stage of their education, and promoting the development of their personal identities and capacity for best learning, is integral to our planning and operation."

Currently, only boys stay on until year 10 at the Cowper campus.

High school girls learn at the Brisbane Street site, and years 11 and 12 have co-ed classes at both sites for core and elective subjects.