It's also excessively common for a teacher with twenty to forty students to more or less expect kids to all learn the same way, pick it up from the book and written exercises, and not really need an explanation. My son ran into situations where he said he didn't understand what the math book said and asked the teacher to explain it to him and the teacher picked up the book and read to him the exact thing he wanted her to explain to him some other way because he hadn't understood it.
Another thing that happens these days is that a game of telephone has gone from "there are lots of valid ways to understand things, so don't punish kids who get the right answer with a different method than the one and only way you've been told to teach" to "teach every kid a half dozen different analogies and algorithms for every concept and punish them if they don't know which one you want them to use."
