If someone is anorexic and has lost weight should i tell the principal?

Don't go behind some: Don't go behind someone's back. To begin with unless you are professionally trained and know details about this person, you are not in a position to make a diagnosis let alone report it. Other disorders including celiac disease may cause weight loss. It would be best to approach this person with compassion and empathy, gain their trust and ask if they might talk to the school counsellor or social.

Anorexia: Anorexia is not an easy disorder to diagnose so make sure you are clear on what you are assuming. I would recommend talking to the person directly and encourage her or him to seek help. If you are convinced this person is anorexic and if you think this she or he is in danger you could consider talking to your guidance counselor. Best.