When a school boasts of its academic rigor, it means its students learn a lot and work really hard. Rigor means thoroughness and exhaustiveness––the gold standard for a good teacher.
You may have heard of "rigor mortis"––which is a medical term describing the stiffness of a body after death. Rigor used to mean stiffness outside of the corpse context, i.e., sternness. The word changes meaning along with our changing standards for what we want teachers to be.