"It is only if we and our children properly remember and understand 9/11 that we can make sure that nothing like it ever happens again," Johnson said at the unveiling in London of a new artwork to commemorate the attacks.
He said the website would help to "provide a controlled demolition" of the conspiracy theories surrounding the attacks on the World Trade Centre and the Pentagon which took place a decade ago yesterday.
Peter Rosengard, chairman of the "9/11 London project", said research in British schools last year revealed "the most incredible ignorance, confusion and misunderstanding" of the attacks.
"Children have to see something to understand it wasn't a videogame, it really happened, it could have been their mums, their dads" among the nearly 3,000 dead, he said.
Rosengard said a survey of teachers found that 90 per cent wanted to teach their pupils about 9/11 but complained they lacked the resources.
The website, aimed at children aged 11 to 16, includes downloadable lesson plans for teachers.
Nat Ogborn, 26, a history teacher at a school in the deprived south London borough of Peckham, said: "A lot of the children... didn't know much about the event, some had only heard a lot about the conspiracy theories, so it's great to have another insight."
One of his pupils, Michael Kuku, 12, said he knew nothing about the attacks until recently.
"I didn't know it happened, I found out about like a year ago."
The artwork unveiled in Battersea Park, south London, is fashioned from steel girders retrieved from the rubble of the Twin Towers and designed by New York artist Miya Ando.