Most schools would much prefer you to arrange routine appointments out of school hours. This is the norm ime.
However, sometimes it just isn't possible though. Emergency appointments should occur as required. Often you will have limited chance to chose a last minute/on the day appointment time anyway. And at my doctors if you want to get a normal appointment slot with a doctor or a nurse - you have to book at least 2 weeks in advance.
It is unusual to get to choose hospital appointment times too ime - I have the same issue for myself as I teach and work in a school. I have managed to normally get the first appointment of the day for my regular appointments - which is at 8:15am or 8:30am - thats the best I can do and I still miss the start of school as a result. Would be the same for a child.
And orthodontist appointments can be tricky too. Round here if you want an NHS appointment then you have to be prepared to have a 9-3 appointment time. There are some school holiday appointment times but that won't always be possible due to how often the child is needed to be seen.
Round here the only way to get out of school hour appointments for orthodontist work is to pay private. I know, I did it for DD - more because I couldn't take time off in school hours so regularly due to working in a school. So even though DD qualified for NHS treatment we have still had to pay in order to get appointments when I want them.
From a schools point of view and attendance marks - you are better going after registration closes for a session, so making sure they are present for morning registration and for afternoon registration - so a late afternoon would work best.