觀無量壽佛經疏 2.94

2.94

奉事師長者，此明教示禮節，學識成德，因行無虧，乃至成佛，此猶師之善友力也。此之大恩最須敬重，然父母及師長者名為敬上行也。

As for "serving teachers and elders": this clarifies that instruction in etiquette, academic knowledge to accomplish virtue, cause and practice without deficiency, culminating in attaining Buddhahood—all this is due to the power of the good friend that is the teacher. This great kindness must be respected above all. Thus parents and teachers and elders are called the practice of respecting superiors.