For you, you just want to get a diploma alone, and getting a diploma is the easiest. As long as you pay, someone will help you get it, and you can get your diploma after waiting for more than a year. If you want to teach yourself, you can try to teach yourself the exam while working, buy your own books without going to college, and then sign up for the exam. This can save the school, basically only need to spend books and exam registration fees, but without the guidance of teachers and review materials, the exam will be more difficult. You should make sure that you have a good intention to study.
This is the general situation. Think about it before you make a decision. By the way, there are many educational institutions that offer self-study classes and give you classes on weekends. This is ok, and the pass rate is relatively high, compared with studying alone. So you can get your diploma earlier.