Different schools have different policies. For instance, many schools in Asia are not legally required to compensate staff for sick days.
Many schools will allow you a certain number of sick days per year, while others have a system in place where you need to work with your colleagues to ensure that your classes are covered. Usually, there will be information about sick leave provided in your contract.
Let your employer know as soon as possible that you are not able to work and be aware that you may be required to submit a doctor's note.