Comprehensive NSW high schools are increasingly doing their own selective testing and some are grouping classes based on NAPLAN results despite research showing that streaming can do more harm than good.

NSW has 17 selective schools and 25 partially selective schools but comprehensive public high schools are using external tests to identify students for academically selective classes while others stream English, maths and science based on ability.

High schools, including Pennant Hills, St Ives, Hunters Hill and Killarney Heights, use the Higher Ability Selection Test, run by the Australian Council for Educational Research, to identify academically gifted students for separate classes.

Miraca Gross, the emeritus professor of gifted education at the University of NSW, said there was an argument that NSW could benefit from more selective schools and primary-level opportunity classes.

This year, a record 13,930 children sat the Department of Education's selective test for 4188 spots.