Whether you have a formal school visit contract, a letter of agreement or terms worked out through email, when booking an appearance, here are some main items you should address, in writing, with your host:

1. Date(s) of event
2. Honorarium: Specify how much you will be paid and when the payment is due.
3. Expenses: When traveling afar, expenses that the host reimburses are typically travel (mile, airfare, train), food and lodging.
4. Number of Sessions: This might include any combination of assemblies, workshops, classroom visits and autographing.  I highly recommend that you require approval of the schedule in advance of the visit.
5. Grades and Size of Audiences
6. Equipment & Physical Set-up: Specify what equipment the host needs to provide, what you will provide and how you need to have the room set-up for your program.
7. Selling Books: If you require a book sale, identify who is responsible for ordering books, collecting money and returning unsold stock.
8. Cancellation clause: This clause is for any cancellation of your visit by the school that is not due to that is not due to an act of God.
9. Inability to perform: This is basically a “reschedule” clause in case something happens to you (illness, death in the family, etc.) where you cannot appear on the scheduled date.
10. Recordings: You might want to require signed permission from you which also specifies that you retain all rights to any recording in any format made during your visit.

Other resources:

Get It In Writing 

Sample Lecture Contract