Lawyers from the Commission for Racial Equality are backing Anthony Gard, 54, a section 11 English teacher at Langdon Park comprehensive school in Poplar, east London. He is also a leading member of the Revolutionary Internationalist League, a Trotskyist group.
He was found guilty of assault and threatening behaviour following the attack on Mr Mawhinney, which Mr Gard and four Kingsway College students committed outside the Houses of Parliament in 1995.
Mr Gard's tribunal starts at Stratford, east London, on January 21. He is set to argue that he was victimised for supporting a campaign by Bangladeshi parents in 1994. He is expected to say the school's governors should not have given him a written warning because the Langdon Park Four Defence Campaign was attacking what it saw as racial injustice.
The campaign opposed the school's support for a police investigation into an attack on a white pupil and called for its head, Chris Dunne, to be sacked after he helped officers with their inquiries. Four Asian pupils were later charged with grievous bodily harm, although none were convicted.
If Mr Gard wins his case, legal experts say it would be the first time the race relations law had been used to protect a political activist from suffering at work for his or her political activism.
Employment law specialist David Cockburn of Pattinson and Brewer solicitors said: "The law doesn't presently assist whistle blowers. This case may indicate that where issues concerned are related to race or sex discrimination, persons who are critical of their employers may have a degree of protection."
In the event of victory for Mr Gard, the tribunal could merely make a statement saying it agrees he was wronged. It could, however, award damages in "recognition of injury to personal feelings."
These would normally be paid by his employer, Tower Hamlets council, but in theory anyone named as a respondent by the complainant would be liable. Mr Dunne, the school's chair of governors Judith Hamilton and the borough director of education have been called to account by Mr Gard, who has taught at the secondary school since 1992.
An inquiry by the school's governors into the 1994 campaign ruled that Mr Gard had supported protesters by attending a public meeting, booking a meeting hall and paying for a stall in a multicultural festival.
The governors gave him a second written warning for "bringing the school into disrepute," which was reduced to a first written warning on appeal in 1995.
Meanwhile Mr Gard had already complained to the CRE, claiming he had been wrongly victimised under the Race Relations Act.
Its lawyers have taken up his case because it could set a precedent and increase protection available under the Act.
Mr Gard was later given a final written warning by governors for his part in the Mawhinney paint bomb attack, again for "bringing the school into disrepute. "