According to Sheriff Leon Lott, the school resource officer was acting in response to a student who was refusing to leave class.
"The student was told she was under arrest for disturbing school and given instructions which she again refused," Lott said. "The video then shows the student resisting and being arrested by the SRO."
So she refused to leave class when it was over basically.
The comments on the article linked are so ignorant, my god. Didn't quite look to me that she was "resisting", more staying still than anything, which in no way warrants this... (Neither does refusing to leave class like...)