Take a look at the original sentence again:
Fighting the urge to crawl under her desk, Adriana looked at the
clock, there was still a half hour left
of class, more than enough time to do her poorly prepared, ten-minute speech.
You chose to fix it the correct way:
Fighting the urge to crawl under her desk, Adriana looked at the
clock; there was still a half hour left
of class, more than enough time to do her poorly prepared, ten-minute speech.
Adriana looked at the clock is a main clause. There was still a half hour left of class is the second main clause. A semicolon is a good choice for joining two main clauses that are closely related in meaning.