If you must ask for time off from your internship (which does happen), here are some quick rules to follow:
1. ASK for time off, don't assume you have it.
2. Don't TELL your employer you are taking time off, always put in a request.
3. Plan ahead! You should be asking for time off with at least 3-4 weeks notice.
4. If something comes up and you must ask for time off, write an email, or speak directly with your boss. Apologize for not asking for time off earlier.
5. Fix The Problem. Suggest times when you will make up your hours.
6. Remember, your internship is a commitment. You can't take off for everything and everyone. You might have to move things around in your schedule to accomodate the internship.