Student misbehaves, she is asked to leave numerous times, she refuses. She escalated the incident by refusing to obey those numerous orders. No one else escalated the incident up until the moment she was being forcefully removed.
From what I've seen of the three clips, this a what I consider to be a plausible position that could possibly be taken by those who are defending the officer.
When the officer attempted to remove her from the chair, she arched her back and stiffened herself to get away from the hold the officer was attempting to put on her. By doing that, she transferred the fulcrum point of the chair toward the back of the chair. The officer followed her backward motion to try to keep his hold on her, lost his grip and, with the backward momentum the student initiated, she fell over. While she was still struggling with the officer on the floor, the officer then got a hold on her and managed to drag her away from the chair. The momentum it took to remove her from the chair as she continued to forcefully struggle against the officer's efforts propelled her across the floor.
The point being, she strongly resisted every effort by the officer to remove her from the classroom. Her strenuous efforts to resist being removed from the chair directly contributed to her falling over more so than the officer's tactics.
Therefore, the officer did not intentionally throw her to the floor. The officer did not intentionally forcefully fling her across the room. The student's maneuvering to escape the officer's hold on her caused her to fall over.The force required to overcome the resistance the student was putting up was the determining factor in her sliding along the floor.
Allow me to strongly reiterate that this is not my opinion of what transpired. But from looking at the videos myself, and wondering about how the officer's defense may interpret the videos, I think it's not that far fetched to say that in spite of how the officer decided to handle the situation, there were compelling contributing factors that the officer's defense could use that pointed to how the student's strenuous efforts to resist her removal was the main cause of her falling and of her being slid across the floor, and that the officer did not intentionally commit to those actions.