Medical appointments are still classed as an absence if they're not there to get their register mark. You'll have your appointment letters, or screenshots of appointment reminders so just send those in and then the mark will be M for medical appointment, which are authorised.
You will still get attendance letters when you hit a certain percentage. Below 90% is persistent absence. Even if they're ALL medical appointments, they are still absences, they are just authorised absences, and so will bring your kid's overall percentage attendance down, and if this goes on all year they will be classed as a persistent absentee all year, which is not good. Not many kids fall into that category by the end of the year usually. Most have a run of bad luck with illness or appointments etc during one short period of the year earlier on, then their percentage attendance will gradually go up as they attend every day from then on.
Illnesses also count towards your child's absence figures, but it is authorised absence. If your child is a "persistent absentee" then they will be discussing them at regular attendance meetings because they'll be on the radar, and they'll be looking closely at the reason for the absences. It is in your interest to try to provide evidence for both medical appointments and illnesses, as then when it comes to discussing how they will manage your child's attendance, they will not be able to fine you or refer you to EWO, and it will be clear that you have fulfilled your duty as a parent to comply with their attendance policy by submitting the evidence of the appointments.
As it's on the radar, though, they might call you to ask you if it's possible to arrange the appointments at different times so that it isn't impacting the whole day. Both my children had millions of orthodontist appointments and they were a royal pain in the arse, both for me and my work, and for them and their education. But it is what it is. I tried get afternoon or early morning appointments so they only missed the end of the day, or once they were in sixth form during free periods. Schools will expect that you do your utmost to make sure your child can attend for as much of the school day as possible on a day they have an orthodontist appointment, exactly the same as an adult would be expected to return to work after a similar appointment. It's not on to take a full day off for one appointment, especially if there are going to be many of them. You need to plan for a way around this going forward.
Schools have a duty to inform parents when their child's attendance is low. They NEED to send out those letters so you as a parent are aware that it may be having an effect on their attainment and progress. Imagine if your child wasn't doing well in, say, biology, etc and you tried to blame poor teaching, or worse, your child, for not trying hard enough. The school can look and see that they always miss biology lessons because they're at the orthodontist.
Just work with the school. They are trying to do the best for your child and they want parents fully informed, and to work with them, for your child. Hence the letters with the percentage absence figures. It will probably help if you call them, and rather than be all defensive and snotty with them, say that you're worried they have a low attendance level, and ask what they suggest you do about the orthodontist thing. You'll need to have phoned the orthodontist first and explained to them that it's impacting their education and request first or last clinic appointments at least some of the time so you can tell school you've done your best to arrange better timings.