Teacher shortages in NSW mean some locked down students are getting minimal supervision as they learn from home, the union says.
NSW Teachers Federation president Angelo Gavrielatos says "alarming" new figures provided to parliament show some teachers are dealing with classes with up to 45 students.
"This is an example of the very real impact teacher shortages are having on students and teachers in schools from the inner west of Sydney to the far west of NSW," Mr Gavrielatos said in a statement on Monday.
"As we prepare for schools to go back next term, it is a stark reminder of why teacher shortages must be addressed as a matter of urgency."
The Gallop Inquiry into the work of teachers warned in February that uncompetitive salaries and unsustainable workloads were contributing to teacher shortages.
"We know teachers face an enormous challenge meeting the needs of students after a long period of remote learning.
"We cannot afford to have shortages of teachers now and in the years ahead as we try and get all students back on track with their learning," Mr Gavrielatos said.
The industrial award covering the salaries and conditions of teachers was due to expire at the end of the year and Mr Gavrielatos said teacher shortages would be "front and centre" when negotiations begin next month.
In its recent submission to a national inquiry into initial teacher education, the NSW government warned of growing shortages of teachers, with the worst impact in rural and regional areas and in secondary schools.