Take a look at the original sentence again:
Tracy always knows the right answer in English class but is too shy to
volunteer whenever Miss Simmons asks a question,
Tracy's hand feels as if it weighs 1,000 pounds.
You chose to fix it the correct way:
Tracy always knows the right answer in English class but is too shy to
volunteer; whenever Miss Simmons asks a
question, Tracy's hand feels as if it weighs 1,000 pounds.
Tracy always knows the right answer in English class but is too shy to volunteer is a complete sentence. Whenever Miss Simmons asks a question, Tracy's hand feels ... begins the second complete sentence. You are always correct when you use a semicolon to join two complete sentences that are closely related in meaning.