Mr Harris, head of Serlby Park comprehensive, Nottinghamshire, and four of his pupils had flown to the Netherlands to observe a sex education lesson for 13-year-olds.
The Dutch teacher showed the class a cartoon of a semi-naked couple. The man had wild hair and a large erection. The woman slipped a condom over his penis.
Then the video showed a cartoon close-up of a vagina. A cartoon finger began rubbing the clitoris. Fifteen-year-old Adam, from Serlby Park, raised his eyebrows at the girl sitting next to him. "Oh my God," she mouthed back.
"It's like porn in cartoon form," Adam spluttered afterwards. "Fair enough if they show them, like, missionary. But they don't want to be teaching them the Kama Sutra."
Mr Harris and his pupils were taking part in a Channel 4 documentary, Let's Talk Sex, presented by Davina McCall. She acknowledged that the Dutch example was extreme, but highlighted the British teen pregnancy rate of 31 per 1,000 births, compared with the Dutch rate of seven.
"I'm not saying we need to jump in the deep end," she told The TES. "But we need lessons in how to conduct relationships. We need to teach teenagers about respect and boundaries and emotional health. All those things build self-esteem."
Back in England, Mr Harris tried to implement at least some of the Dutch techniques. He introduced peer-mentored sessions, in which teenagers taught classmates to place condoms on dildos.
It was not popular with everyone: a few parents complained. "My 11-year-old daughter is enjoying her childhood. You've no right to destroy that," said one.
The problem, said Mr Harris, is that while those who complain are in a minority, they can be very vociferous. Ms McCall said she thought that those who supported a more open approach to sex education should be equally vocal.
"We want children to wait until they're in a loving relationship, where sex can be magical and sweet and romantic, not a drunken mistake after binge-drinking on Friday night," she said.
* Let's Talk Sex, C4 tonight at 8pm.