The best thing to do is be good-natured about it. The professor may tease you a little, and the class may laugh, but this laughter is more often than not nervous rather than malicious. Sort of, but its not as bad as you think. In most cases, youll have something like three or four hours to answer two or three fact patterns. A fact pattern is a heinously long story, the point of which is to get the student to extract each possible theory and define, apply and conclude it.
Many schools keep old exams and even model answers on file at the library.
Most professors want the kitchen sink on their exams. Others want a tight, clear exposition on the relevant facts and points of law. The general rule here is to practice taking full length-exams well before exam week. This is where study groups are helpful. Each person in the group should take the practice exam alone, and then get together to compare answers.