Take a look at the original sentence again:
Susan ducked down when she saw Mr. Hayden, her English teacher, walk into
Tito's Taco Palace, unfortunately, a burrito
wasn't big enough to hide behind, so poor Susan had to explain why she had
missed his class again.
You wanted to fix it this way:
Susan ducked down when she saw Mr. Hayden, her English teacher, walk into
Tito's Taco Palace; Unfortunately, a burrito
wasn't big enough to hide behind, so poor Susan had to explain why she had
missed his class again.
A semicolon is an excellent way to fix a comma splice. You must be careful, however, that you don't capitalize the word that follows the semicolon. Unless the word following the semicolon is a proper noun, one that is always capitalized, you should leave it with a lower case letter.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.