If you are getting an excuse for missing school from a doctor, parent or someone else authorized to write one, they will need to address the authority figure that the excuse will be given to. This might be a teacher or professor so you'll need to know their name so it can go at the top of the doctors note or letter from someone else.
Your excuse for missing school needs to be something valid and something that was out of your control otherwise you should not have been absent. If you are getting an excuse from a doctor then you should be covered. If it's from your parents or someone else qualified to give you an excuse for missing school you'll need to have a good reason for being gone such as a funeral, vacation, or family emergency. This reason will be explained in the body of your excuse and clearly stated so it is easily understandable.
The end of your excuse for missing school should be signed by the person giving it to you. This step is very crucial because if the person forgets to sign it then it may appear to be fake and that is the last thing that you want to have happen.
It never hurts to have the person giving it to you put their phone number on there as well, in case your teacher or professor has any additional questions so that they can be contacted. They will rarely do this but it adds some extra reassurance that your excuse is genuine and then if they truly do have any questions they can call the person who wrote it for validation.