Everything in writing, even if it is a telephone call with the school follow it up in writing to confirm what was said. I installed one of those apps that records all calls meaning I could transcribe the phone call but obviously not use that in the email but very similar language to confirm what was said.
Get their bullying policy which is almost always on their school website under policies. Also look at their safeguarding which is the key word here, your daughter is entitled to feel safe whilst at school. Pull them up on where they are failing. Don't give up, every time there is an incident you need to be all guns because sometimes with schools that is all they care about.
For the park incident with taking her skateboard, police. Let the little shits know how serious it is. There is often a police officer assigned to a school too. Diarise every event, have a specific notebook or app for notes on your phone (better as accessible at all times) write everything that happened, what you then did and what school did or didn't do. Let the police know she has ASD and you think she is being targeted because of that.
Re school, if there is another school she could attend consider that. Moving a child for year 10 or 11 is almost impossible due to GCSEs, (thinking of your eldest) even if they are the same exam boards it does not mean they are taught in the same order.
And for your lovely daughter, not that this massively helps but tell her that there must be something very special about her that makes her their target. The opposite of love is not hate, it is indifference. There is something that they want to reduce in her, it can be a jealousy of something about her. They seek her out, notice her. So she clearly has something that puts her on their radar.
Both my sons did martial arts from primary school. Great for discipline and teaching them wrestling and grappling so it isn't something new when thugs try it in school. Came in handy the first few weeks of year 7 when another year 7 kid thought that my son looked like an easy target. Boy was he wrong, he wasn't hurt by my son just mortified and embarrassed that my son was so good at deflecting punches and kicks. I wish I had been there. It ended when my son had hold of his foot at waist height and the bully was hopping around like an idiot trying to keep his balance. Ds could have easily lifted his foot higher and swept his other leg but didn't want to be seen as the aggressor. Other kid got isolation, mine did not.