Robin Sivapalan, who works at Quinton Kynaston secondary school in St John's Wood, north London, organised the demonstration to coincide with the Prime Minister's visit earlier this month.
Pupils joined members of trade unions and the Stop the War coalition at a rally outside the school to protest about privatisation and the wars in Iraq and Lebanon.
Mr Sivapalan, 25, who has worked at the school for a year, said he was suspended for "breaching confidentiality" and will appear before an internal disciplinary panel later this term, when he could be sacked.
However, a spokesman for the school said his suspension was for a number of different offences stretching back to April. "We have suspended a teaching assistant pending a disciplinary investigation into a range of activities over a six month period," he said.
Campaigners, led by Unison, the public services union, have launched an on-line petition calling for Mr Sivapalan to be reinstated. They say his suspension is an "attack on freedom of speech and the right to protest".
Mr Blair was visiting the school with Alan Johnson, the Education Secretary, to announce the first wave of "trust schools", the new generation of state-funded secondaries run by businesses and charities.