the problem with this little gem is the fact that if the principal denies you excused absences on doctor and dentist visits it could land you in court, or jail.
Man I hope that happens to you and I get to sit on the jury, because the principle would have his butt in a sling. Do you HONESTLY believe that a principle is going to tell a parent "No, you may not take little Sally out of school to go see the dentist and have the absense excussed." Dude one phone call to the school board and that priciple is looking for a new job and that's as far as it would end up going and you know it.
I do however believe that principle would be right to tell a parent, "Sorry but if your taking little Sally out of school because it's the opening day of fishing season and she just really wants to go, well that will be counted as an unexcussed absense and will reflect poorly on her transcript, but that's up to you."
Understand that laws are for the most part based on what most reasonable people think is right. Bad laws get over turned and tossed, good laws stay around longer. Seeing as this law is over 30 years old, must be some solid reasoning behind it.