@anotherRat if you had a 39°C fever and went to school normaly, you not only put yourself in danger, but everyone around you in danger of catching your sickness as well.
39°C may not be a "dangerously high" fever(it would be at the lower end of high grade fever) and would still be the point at which you should think about seeking medical care, if not at least resting(especialy as a child/teenager) Heck, where i live sending your child with 39°C fever to school can actualy be considerd negligent child welfare endangerment(depending on severyity of case)
Its also a cultural thing, same reason why People in asian countrys wear masks even if its just a harmless cold, its "basic not being an asshole" thing