The WPPSI is only an intelligence test. It is not designed to test achievement. That said, it is usually the case that kids who test well in terms of achievement (especially when they haven't been taught) will also test well in terms of intelligence.

If you are using the test to take to schools, you really should check with them first to see what tests they accept. You don't want to spend your money on a test that they will simply disregard.