I've name-changed to protect my privacy here, but I am a regular.
The teacher will have no input into the timing of some of the appointments she has to attend in relation to the child. She will be legally obliged to attend meeting when called.
I am currently looking after a close family member, with a view to the child concerned living in my home permanently. The child is what is known as a "Looked After" child, having been downgraded from being on the child protection register.
We have had to attend regular multi-agency meetings involving social work, psychiatrists, health visitors, mental health teams, the police and so on and so forth. All of these professional people have to somehow manage to find a time they can all meet. As you will imagine, all of them will have busy schedules. I have been at meetings with at least twelve people all there to discuss one small child. When we had to go to court regarding the child we couldn't choose when to attend.
YABU in being angry at this teacher, who I have absolutely no doubt would rather not have to attend meetings, doctors appointments etc. Believe me, it's not a good reason for a skive off work!
Your son's school needs to make appropriate arrangements to cover the teacher's absences. That is who you should be aiming your annoyance at.