A former Berwick Secondary College teacher has been charged with 70 counts of child-sex related offences after allegedly upskirting his students over four years.

Police have been unable to identify the victims, who are believed to be between 11 and 17 years old.

The school wanted to narrow the pool of students who might be potential victims but police told them it would not be possible to identify the children in the upskirting images.

The 61-year-old man was charged in January with making, using and possessing child pornography, and an indecent act with a child in his supervision and care.

"It is believed the alleged offences occurred at a school and police have been in liaison with the Department of Education and Training throughout the investigation," a police spokeswoman said.