So they had no reason to request her to leave a public school therefore the "criminal trespassing" charge is bogus and since the charge is bogus the use of force was illegal.
And that is the rest of the story the supporters of the badged brutes would prefer not to know.
Why not? School grounds are not normally open to the public, and non-students usually have to be invited to be there. And on just about every ticket I've ever seen, whomever is inviting the person reserves the right to enforce its rules, and to eject anyone who doesn't comply.
School policy might apply to school visitors, do you have a copy we can consult?