Get signed off

Hi,

"You can miss one class without giving the teacher reasons for your absence, but if you miss another one, you have to bring proof that you got signed off sick by a doctor." Is this correct? I'm curious about the use of 'sign off'.

"If you fall ill and you have an exam coming up, you have an option to withdraw from the exam/cancel your enrolment and enrol for an exam on a different day. " I'm pretty sure that I can use both 'withdraw' and 'cancel' here, but I was wondering if I could also use something like 'log out' (This doesn't seem to fit the situation. I'd be more likely to say this when logging out of a website.). Perhaps you can think of a different alternative.

Thank you.