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.