The footage, a blurry clip possibly from a mobile phone, aired on popstar Allen's show on Tuesday.
In a segment called "funny internet clips", Allen introduced the video to the audience, saying: "Now it's my favourite, it's kegging - pulling someone's trousers down in public when they least expect it. It's very childish, but very funny.
"CJ from London has sent me one, it's kegging a teacher."
Andy Brown, an executive member of the Association of Teachers and Lecturers, condemned the video, saying: "If she (Allen) doesn't understand the responsibility of what she is doing in the media world, then she should not be allowed to do that."
General secretary of teachers' union NASWUWT Chris Keates said: "We can only imagine what that teacher must feel about what happened to them at the time. Then [it has been] made hundreds of times worse by the fact that it is posted on the internet - and has now been shown on the BBC."
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