Tell the school then bud because it won't help matters especially when they get onto harder stuff and will need to make substitutions for other problems. I am currently at uni doing a mechanical engineering degree and I have core maths modules so I'd like to think I know my stuff. From experience, you have to nail these things on the head right from the beginning to I would go to the teacher and tell him/her your child has a problem and to help her sort it out. If the teacher is at fault for not being able to explain properly try addressing the head of department.