Since it's publication in 1826, James Fenimore Cooper's The Last of the Mohicans, one of five books in the Leatherstocking Tales, has enjoyed great popularity both in American and abroad. Arguing that it is not a classic is a subjective exercise and one that needs a strong thesis and plenty of evidence to support. It also is dependent on how one defines a "classic." You may want to look at several different definitions and examples. On one hand, a classic, whether it be a book or a piece of music, is something that stands the test of time. By this definition, Mohicans is very much a classic. It has been constantly in print, is often taught and read in schools, and has been adapted into numerous films. It is firmly part of the American literary canon.
However, being a classic doesn't always mean that something is good. Many have found fault with the book, especially with Cooper's plotting and writing style, which can be hard for a 21st century reader to get through. Mark Twain was, perhaps, the most famous Cooper critic. He wrote an essay called "Fenimore Cooper's Literary Crimes," which is worth looking at. Again, I think it's important to define classic and then demonstrate why you don't think the book merits that status. Two questions to ask are: Does the novel still have anything to say to a contemporary audience? Does the novel tell us anything about the time in which is was written?