Kelly Giles was waiting to pick up her child from Merton Abbey Primary School on March 12 when she was confronted by Keyleigh Gore, who claimed Miss Giles had called her fat and insulted her daughter.
Miss Gore pushed Miss Giles who fell backwards. She then started repeatedly punching her.
Sasha Fisher, another mum waiting outside the school, tried to break up the fight, but then also turned on Miss Giles, pushing her in the shoulder and hitting her in the face.
Miss Giles was left with bruising to her lower back, arms and above her left eye.
Miss Gore, 25, from Wimbledon, south London, admitted she had pushed Miss Giles, but said she did so to 'get her off her' after the two started pulling at each other's hair. Miss Fisher, 21, told Wimbledon Magistrates Court that she had broken up the fight, and only hit Miss Giles because she feared for her safety.
Following a three hour hearing, both mothers were found guilty of assault by beating. Their claims of self-defence were rejected.
John Brummell, chairman of the bench, said: "The issue in this case is self-defence. We found Miss Gore was the initial aggressor. She pushed Miss Giles on to a low wall and proceeded to punch Miss Giles.
"Miss Fisher initially intervened but then subsequently assaulted Miss Giles by punching her. The extensive injuries suffered were as a result of these two assaults."
The two women are awaiting sentencing.