There’s no replacing language teachers. They’re there to help you learn better, faster, and with more confidence. They will show you what to learn and how to learn it, and they won’t allow you to fail!
With teachers comes lesson organization. It’s one thing trying to learn a language through a series of unstructured individual efforts and a completely other thing trying to do the same through a string of meticulously planned activities.
Finally, although it might sound a bit silly, language schools ooze a learning atmosphere. When you’re trying to learn a language on your own at home, you can get easily distracted. On the other hand, when you enter a language school, you know you’re there with a goal, and that goal is learning English.