Quote Originally Posted by Rtyom
The letter change stands for different senses of the word "учитель".

Учители are people who give you advice on something, who help you in your life because they are old and expereinced.
Учителя are people who teach you, mainly, in school.
Thanks, I would never have figured that out. So there are two possible answers depending on the situation. Wish the textbook would put a note on the matter.