A behavior contract is a plan of action that is negotiated between a client, child, spouse, etc. and concerned others. Often the behavior contract will include both short- and long-term goals. The contract objectively specifies what is expected of the person and the consequences that follow behavior.
Essentially, behavior contracts state:
1) what the rule or expectation is
2) what reward or reinforcer is available for desired behavior
3) outcome/consequence is delivered for undesired behavior
While it is easy to "assume" that students "know" the rules, it is often better to ensure that all expectations are clearly outlined. Sometimes putting them in writing (or using visuals) can be very helpful.