Calling teachers by nicknames helps the students have a bond with their teacher. Studies show that in classes where the teacher is liked, learning is up by 500000%. Using nicknames save time, that we could use talking about trips to Quebec, or maybe something related to calculus (unlikely but possible). For example, if I were to say Mr. Hammer, that takes up valuable time. However, Brother Joel has one less syllable, thus saving time for studying Calculus.