Can cognitive impairment from bipolar disorder affect one's grades and overall intelligence?

It should not: Bipolar disorder is a mood disorder with swings of depression and mania. It should not affect intelligence, but it can affect school performance. Many very bright people fail in school due to mood problems. For others, failure in school causes mood problems. These issues need to be addressed by psychologist/psychiatrist as appropriate.

Subtle question: In mania, people subjectively feel their performance is superior when actually it is not--it becomes degraded by distractability and flighty thinking. Yet underlying cognitive capacity remains intact. Mild mania (hypomania) provide boosts of energy that multiply the effect of intelligence even though, once again, the underlying capacity remains unchanged. Untreated bipolar reduces net performance.