Answer:
It’s time for the class to end, but Mr.Johnson doesn’t realize... (the rest of the sentence was cut off)
Explanation:
The semicolon should only be used when there are two different subjects. In this case, there is only the fact that class is supposed to end, but Mr. Johnson doesn’t realize something.
There are always some cases where you dont even need a comma, but it doesn’t really fit right. If you were to say that sentence out loud, it would commemorate to use a comma, by natural spacing.
