Yes, many people have done that - people who have shown their dedication and brilliance. But it is not a general case, as almost all the students opt for the other way of joining some or the other coaching classes. Some only take classes on the subjects they are weak at. I'm not discouraging you, I believe everyone can but these are the real facts.
An instructor helps you in making all your basics clear with his knowledge and experience. He shares the materials and his notes, that have helped his past students. He is like a guide that shows the path to a maze runner. We, here at Spanedea, help you on the topics that you are facing difficulties in. The interaction is one-to-one; we (from our own experience) make sure the student isn't left with any dounts at the end.