At least in The Netherlands, about 15 years ago when I was in high school, timetables where set for a whole semester, but absent teachers where quite normal meaning some lessons where canceled. Replacements where not always available and where usually only deployed when it was known a teacher was absent for extended time.
As a teen this was great because sometimes you could stay in bed longer, be out earlier and not having to carry the extra books (not sure if that still is a problem now).
edit: I just remember we even had call trees to call your classmates in the morning if classes got canceled. This was before the internet, making it not 15 years ago but at least 20, probably more. I'm getting old...