So sorry you had this experience, OP. FWIW, my DD's school is amazingly supportive to kids with ASD and this would never have happened there.
One poster upthread said: "I personally don't think that there a different treatement for kids with ASD if she is in the mainstream school. They expect all children to obey school policy."
That's simply not true.
DD has ASD, and when heading into a meltdown is able to leave class, go to a special room (for the SEN kids), has a '5 minute pass' so she can leave early to avoid crowds, can have her lunch in the library, not attend lessons she finds distressing (eg music, too loud).
She only got this support once under the care of the SEN team at the school. Does yours have similar?
But yes: they are treating ASD as if it is elective, or an illness, when in fact it is not. So many people just don't understand it, especially in women and girls, and this includes teachers and governors.
So you might want to research best practice in this area in your letter to show how your DD's school is letting her down by trying to force her to behave like a NT child, or within the frameworks that they have designed for NT children.
She's not 'behaving badly'. She is autistic and they are legally obliged to make reasonable adjustments to enable her to attend school despite her disability. Suspending her, particularly when you were there, is not a reasonable adjustment. Nor is it a proportionate nor informed response when faced with an autistic meltdown.