The Grey Coat Hospital girls school in Pimlico has suspended 29 pupils after they posted malicious comments about a teacher on social networking site Facebook.
The pupils at The Grey Coat Hospital CofE School, in Grey Coat Place, wrote the offensive material after joining a group called The Hate Society on Facebook.
The girls were suspended for between two and 15 days.
Headteacher Rachel Allard said: "While the offending material has been removed from the website, the school's decision to exclude these pupils temporarily was not taken lightly and has been designed to send a strong message to our whole school community that we do not tolerate such behaviour.
"The vast majority of parents who have been to see me about this incident are supportive of the school and understand why we have taken firm disciplinary action."
A Westminster Council spokesman added: "We support the school's decision to temporarily exclude these pupils. We are providing advice and support for the school during this time."