The classroom also had glass windows looking outside, glass panes on the doors, and a glass partition at the back of the classroom above the students' pigeon holes that looked out towards the tuckshop, a court has heard.

David Kisun, 71, is on trial in the ACT Supreme Court charged with three counts of indecent assault on a 9-year-old boy in the 1980s. He has pleaded not guilty.

Prosecutors contend the teacher sat the boy in the back row and used the opportunity when students were working on individual tasks to reach into the boy's shirt.

It's also alleged Mr Kisun made the boy stand at the back of the classroom, and touched him under his grey school shorts.

On other occasions, the former student said, he was asked to stay behind after class and sit on the teacher's desk, and Mr Kisun would reach into his shorts through the leg.

The court has also heard the boy's brother say that Mr Kisun touched him in a similar way while a student at Marist.

Mr Kisun, who gave evidence on Wednesday, said there was "no chance" he would have been able to touch the boys.

"Someone would have definitely seen me do it," he said, pointing to a busy school and the glass windows.

He told the court there were some 400 students in the school at that time and more than 30 students in his classroom.

He denied he set the students individual work, and said that was "something [the complainant and his brother] copied from the Royal Commission".

The students worked in small groups, he told the court.

And he was always watched and asked for help.

"We had 60 sets of eyes in the classroom."

The former teacher called it an "impossibility" that he would have been able to reach under the boy's shirt with one button undone and a tie loosened as had been alleged.

Mr Kisun said he had not put the brothers in the back row of his class because they were good students, and instead said he used a ballot to mix up the seating every few weeks.

In cross-examination, prosecutor Trent Hickey suggested Mr Kisun was making these things up now to distance himself from the allegations, which the former teacher denied.

"Were you sexually attracted to some of the boys in your classroom?" the prosecutor asked.

"I was married then," Mr Kisun replied. "No thank you."

He said he never kept students behind after class at recess or lunch because he was in charge of the tuckshop, and had to leave to supervise the students buying food.

Another former teacher gave evidence Mr Kisun was always supervising at the tuckshop. "He was there every single day."

The trial continues.