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This is standard procedure. Your parent or legal guardian will need to call the school and explain that you cannot or will not be coming in that day. Most schools only require your parent or legal guardian to say that you will not be coming into class that day. Some stricter schools, however, might require a specific excuse, so be sure to check your school's handbook. The idea behind this practice is to reduce the amount of unexcused absences and to keep tabs on the sorts of illnesses going around. Many schools require a parent or legal guardian of a student to call in regardless of that student's age, but some will allow a student who is a legal adult (age 18 or older) to make the phone call on his or her own behalf. For prolonged periods of illness, your school might require you, a parent, a guardian, or a family member to bring a signed doctor's note stating that you are legitimately ill and needing more time to recover. A physician's note becomes necessary after your illness extends beyond a certain period of time. The exact amount of time can vary by district, so you will need to check your school's rules to know at what point the note becomes a necessity. This amount of time usually ranges anywhere between three to ten days, with ten days being more common.
Persuade a parent or guardian to call in. Call in yourself if the school allows it. Get a doctor's note.