Bonjour,
Voici mes réponses, en espérant que cela t'aide...
1. Sorry I can't have lunch with you. I'm (or I am) seeing my mum.
2. Here comes the principal. Look! He's (or He is) wearing a wig.
3. Ferris' not (or is not) really sick. He's (or He is) only pretending.
4. You're (or you are) very pale! You're (or you are) feeling okay ("Are you feeling okay?" would be the preferred way of asking)?