Signing in and out of the school is done at the reception desk, just through the front door.
If you arrive at school after 0845, you will have been marked absent. So you will have to sign in or no one will know you are in school. The school needs to know if you are in, in case there is a fire or the school has to be evacuated for any reason.
If you need to leave school during the day then you need to sign out, unless you are just going out for lunch. When you sign out you have to have a note or appointment card explaining why you are leaving or your parents must phone in to the school before you are due to leave.
If you are returning to school later that day, you must sign in at reception.