Take another look at the original sentence:
You wanted to fix it this way:
To add a comma between student and when would cause an equally bad problem, a comma splice. A comma splice occurs when you have two complete sentences joined with a comma alone. Tajuana is a very conscientious student is the first complete sentence. When she makes a B, she is so upset ... begins the second complete sentence. The spot between student and when needs a stronger break than a comma alone.
You might want to consult the rules for fixing comma splices and fused sentences.