It is a play on a couple of different terms. In most American schools A is the highest grade one can get. A class which is an "easy A" is one which a student can get a good grade without having to work very hard. The title is also a play on the term "easy lay" for a person, particularly a woman, who is very willing to have sex (ie, you don't have to put a lot of effort into convincing them to sleep with you.)
Finally, the title is also a reference to the Nathaniel Hawthorne novel "The Scarlet Letter" which Olive is reading for school. The novel follows Hesther Prinne, a woman living in Puritan New England, who becomes pregnant after an extra-marital affair. As punishment, Prinne is forced to wear a red A, for "adultery", sewn onto her clothes.