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