Almost every institution of higher learning has a policy which states that students are prohibited from turning in a piece of work to more than one class without the express permission of the student's instructors. When students do so without permission, they run the risk of being found responsible for violating the institutions academic integrity policy which carries consequences ranging from a failing grade to expulsion depending on the severity of the offense.
The keyword in this statement is an ORIGINAL essay that you wrote. The essay is your property, not your professor's property, and most definitely not your institution's property. Most classes allow you to sign a waiver stating whether or not they can use your paper for their own purposes. Say no. If a professor wants you to submit a paper for a particular assignment, you are to submit a paper that you wrote yourself. It doesn't matter if you wrote it last week or three years ago, it's still your paper to submit. It also doesn't matter if you published the paper on an online blog, or submitted it to three different classes, it's still your paper; it's your property.