Brown, 51, was taught biology by Talbot, who worked as a teacher before his TV career took off, and told the jury the defendant showed his class a gay porn film in school.
Talbot, 65, denies 10 offences of indecent assault on five boys while working as a teacher between the late 1960s and the early 1980s and is on trial at Minshull Street Crown Court in Manchester.
Brown told the court he was a pupil of Talbot at Altrincham Grammar School from 1974 to 1979.
Wearing trainers, jeans and an open-necked shirt, Brown said in the 1970s the all-boys grammar school was a violent place - but Talbot was one of the few teachers who did not physically hurt pupils.
Mr Brown said he remembered "in particular" two or three biology lessons given by Talbot when he was an 11-year-old boy.
The witness said: "Very early at school, I would not have been there a long time, Mr Talbot asked all the class if any of us had ever masturbated. We were 11 years old at the time.
"He went on to explain how to masturbate, how you should masturbate and the following lesson he asked who had masturbated."
Neil Usher, prosecuting, asked if there was any response from the boys.
"Nervous giggles," Mr Brown said. "He wanted to know who was successful in the masturbation. He asked boys to raise the hand."
Talbot denies all the alleged offences.
Mr Brown said in further biology lessons, Talbot would approach the boys who had raised their hands, the ones with "bum fluff on their lips" and have "private words" with him, "murmuring" in their ears.
The witness continued: "I can remember sitting there wondering, 'What is he saying to them?'
"They were all the same age but they were a foot bigger than us."
Mr Brown then told the jury of a second memorable biology lesson with Talbot.
He said the class went to the science block, but instead of the usual classroom went into another with a projector set up to show the class of 11-year-olds a film.
Mr Brown said: "A three minute film on a Super 8 projector, of a guy walking into a room dressed in denims, sits on bed, takes trousers down and masturbates.
"The film was about three minutes long.
"It was only a few years later I realised it was probably a gay porn film. It wasn't a sex education film. It was years later I realised it was wrong to show us that film."
Mr Usher asked how old the witness was at the time.
Mr Brown replied: "Around 11. About the time you would have had sex education in school."
Much of Mr Brown's other evidence can not be reported for legal reasons.
Talbot became a TV regular on the floating weather map in Liverpool's Albert Dock for ITV's top-rating This Morning show after his teaching career had come to an "abrupt end" in 1984.
The jury heard he resigned his school job after allegedly propositioning a pupil at his home and the headmaster was informed.
But the court heard while working as a teacher Talbot was a "chancer" who used his "boundless energy" and "extrovert personality" to abuse some of the boys he taught, four of them teenage pupils at Altrincham Grammar School.