Here's a BBC article on the case. Obviously 1) there was more to this than just a few posts on Whatsapp, and 2) the situation did get out of control for a brief while and the police were called. Probably similar to common "affray" or public disorder cases.

>The school said it had "sought advice from police" after a "high volume of direct correspondence and public social media posts" that it said had become upsetting for staff, parents and governors.

https://www.bbc.com/news/articles/c9dj1zlvxglo