What are valid reasons for missing class in college?

Arguably the most valid reason for a student to miss class in college is if there's a scheduling conflict with a religious observance. That applies even to exam days; the teacher must arrange for the student to take the test at another suitable time.

A death in the family and jury duty are also valid reasons for absence. While school policies insist on full attendance, their short list of excused absences also includes illness, such as the flu or a stomach bug. Here, common sense would dictate that it's better for a student to remain home so other students don't get infected.