Windermere independent MLC Ivan Dean questioned the government on the numbers amid its roll-out of year 11 and 12 extension programs to some of its feeder schools.

He said the college had the capacity to provide positions for almost 800 students though enrolments had declined in recent years.

Leader of Government Business in the Legislative Council, Leonie Hiscutt, said colleges would play a critical roll in the delivery of year 11 and 12 courses and give parents and students choice and diversity.

She said Newstead College had 704 enrolments in 2017, 639 enrolments in 2018, and a headcount of 513 students this year.

This is a mixture of full-time students and students who attend the college for particular subjects.

The college's main feeder schools, Queechy and Brooks high schools, will receive year 11 subjects in 2021 and have both year levels by 2022.

Another feeder school, Kings Meadows High School, has already completed its extension program.

Ms Hiscutt said there would be a requirement for all students to complete year 12, or attain an equivalent qualification, under a policy to be implemented next year under the Education Act.

"Implementation will see an increase of students staying at college and extension schools," she said.

"The Tasmanian Government's policy is not aimed at dismantling colleges.

"Colleges are vital in providing a broad curriculum [and] remain critical to the delivery of years 11 and 12 in Tasmania."