Dr. Nate Kreuter (Western Carolina University) uses a student/instructor contract as a way of helping students understand both his expectations of them and what they can expect from him.
As you can see, he uses humor but still makes it clear that he (1) expects the students to do the work, (2) gives them the choice to drop so that others can take the course if they are unable/unwilling to take the course seriously, (3) knows this is a great course (self-fulfilling prophecy here), and (4) realizes that while this may not be a legal document, he expects the students to illustrate their commitment to the course by signing it.
Here's his intro:
Welcome to the class. You’re probably wondering what this class contract is about, so I’ll go ahead and explain. I want to make sure that everyone in this class realizes from the beginning what we will be doing and what my expectations are for each and every student. This document is not intended to discourage you generally or to encourage you to drop the class. Just the opposite. I honestly think this will be the best section of Composition 101 offered in the Department of English this year. But I know for a fact that it will be one of the most demanding. The risks and rewards are great. If you put in the work, this will be a wonderful class. If you don’t, you and I will both be miserable. So, I’ve listed some of my expectations below. Look over them. Carefully. The reading and writing demands of this class are high. But the readings are also unusually enjoyable and the assignments will be, I hope, also enjoyable and enriching. If you can commit to doing this work, go ahead and sign the contract. If you can’t, consider giving up your seat to someone who can. This contract is not all-inclusive but should give you a pretty good idea of my expectations for the semester.
Please note that the course is designed to start very quickly, with a lot of work and reading due up front, and is designed so that the amount of work you have to do will gradually decline over the course of the semester. The hope is that you will have the bulk of the difficult work for this course completed by the time your other classes are becoming especially burdensome at the end of the semester.
The reality of this contract is that it is meaningless. If you sign it but fail to uphold your end of the bargain, I can’t do anything to you. I can’t have you expelled, or even lower your grade. However, I am absolutely certain that I will hold up my end of the bargain, and I hope you will too. Our word will be our bond. Welcome again to the class. Sign the contract, do the work, enjoy yourselves, make friends, and let’s never have to speak of contracts or similar unpleasantness from this day forward.