Don't feel bad about using the B word. I should know better, but I used it with my DD's teacher this winter, and the result was exactly what I should have expecte. I said it very gently, but still got the defensive posturing and then she spoke to my DD5, asked her what she found "boring" and then proceeded to tell her she had to be bored and do those things so that she would be ready for 1st grade - now my DD5 thinks that going to school is supposed to be boring because that is how you get ready for the next grade!
When teachers say school is sometimes going to be boring there is a grain of truth in their views, for at least two reason. First, no two children in a 20-student class are in exactly the same place and learn at exactly the same rate, so there will be times when children are being taught what they already know. Second, sometimes children may need to learn some things that they do find boring, whether its the multiplication tables or names and dates in history.
Parents should not expect their children never to be bored, and teachers should not expect children to be satisfied with never learning anything new and always being bored. It sounds like the teacher who emailed the OP is not even trying to meet the needs of the child.
