First off, congrats for beating the national median on your first try. As for improving, most everyone sees improvement if they study (in a class, or with a self-study program). Don't sweat it. If you look at various claims out there, the standard increase for a course is 10-14 points. Before you plunk down some moola, try buying a book (I like ours

), and see what you can do on your own. If you know you need a class to motivate you and make you do the deep work that is required to make a real serious increase, go for it.
Good luck!