Classics appear in all my yearly challenges. I don't do one a month because I usually have so many reading commitments that it's not going to work out. I go for a minimum of 6 instead.
There's various definitions of classics that are bandied about in these forums because it's occasionally contentious as a book club category.
Personally, I would put both The Great Gatsby (on my TBR for 2013) and Clockwork Orange on the classics list. I happily include Vonnegut on mine. I don't actually exclude pulp from the classics tag as long as I feel that it has an enduring value - eg. The Maltese Falcon, Riders of the Purple Sage etc.. It all depends, of course, on what type of education you want. You might want to exclude pulp from the category if it doesn't meet your aims.
I asked someone at work how they would classify "classic" and the reply was that the book would need to be at least one generation old (possibly two) and still have value to the current generation. That feeds into that "enduring quality" aspect. The reason for this is that fiction largely communicates to a contemporary audience. Once that generation passes, the message can become diminished or "dated". A classic becomes one because it continues to communicate effectively to current generations after other contemporary works of that time have faded into irrelevance.
No doubt there's exceptions to this all over the place and I'm certainly not prescriptive on this. But it's food for thought.