Books: You do not need to buy any books for HOL classes. Most of the materials you'll need are available either on the classroom website, or on the Internet.
Age Limit for Professors: Professors must be at least 17 years of age. This is due to the maturity and responsibilities associated with being a professor. Though HOL is ageless, those in positions of authority must be able to handle the responsibilities from start to finish.
Student Teachers: Individuals who are less than 17 years old and wish to teach a class, or students who wish to teach a class but would like to remain students can apply to be a student teacher. Only a limited number of classes with student teachers are approved each year, which means that if you apply to be a student teacher your application may be rejected.
Class Prerequisites: Most classes do not require that you have taken an HOL class before, but HOL has a few advanced courses. READ the class information when posted... if a prerequisite class is required, the class information will indicate such.
Dropping a Class: Just send your professor an email/HOL message asking to be dropped from the class.
Exams: Exams are held essentially the same way homework is handled, and so it depends on the professor. Check the requirements of your classes to see if there are exams.
Homework: Although each professor is different, they generally prefer to receive homework in the text of the email, or posted onto a forum. Check the classrooms of those classes you wish to take; it's generally indicated there. Or, you can check to see if there are any prefects in your class, and ask them.
Number of Classes: Each student may take a maximum of five classes at any one time, so choose your classes wisely.
Vacations: Yes, professors need, deserve and get them, just like in real life. Summer Break: typically July 1-August 31, and Winter Break: typically December 20-January 8ish. Do not email, PM or otherwise bother professors during these breaks unless they request it or deserve it.