I am afraid that I shall not be free till the examination is over.

You should continue until you're satisfied.

If you think you can get away with cheating on the exam, you've got another guess coming.

That would be twenty-seven words instead of four, and while the bare message of the longer statement would be understood, the persuasive force would be lost.

That absolves me from further responsibility.

You will save your father a lot of worry if you simply write him a letter.