On the morning of the day in question, "ring, ring, ring, hello Principals office...good morning this is shamus, shamus Jr. will not be in school today he has a dentists appointment".
Should be the end of it.
shamus
But what about shamus jr's homework for that day? Wow guess what? He's going to be a day behind now because you didn't let the school know in advance, you know when the teachers would have been informed and could have gotten shamus jr's work for the day he was going to be absent prepared for him so he wouldn't fall behind.
There is a LOGICAL reason they want to know in advance, and that one right there is the biggest one. How about if he had a test that day? Well he misses the test, talks to his buddies that night and gets the answers, and cheats on the makeup, IF the teacher allows him a make up test. BUT if the teacher knows in advance they can make a seperate test for shamus jr. or maybe have him take it a day early, whatever. It provides the school needed time to ensure your child doesn't fall behind, and if shamus jr does fall behind who are you going to blame? Not yourself, your going to blame the school for your own screwup that could have easily been prevented by picking up the phone a day or two in advance.
By you picking up the phone the day shamus jr isn't going to be there and having the attitude of "that should be the end of it" you have in FACT caused a bunch of other people a bunch of problems of having to figure out how to keep little shamus jr on track and up to spead with everyone else in his class. Doing what you suggest actually hurts your kids own education and YOU are defeating the entire purpose of sending your child to school.
Bet you never thought of it that way huh?