You have confidence at home when you have notes to study from, but on the exam what happens to your confidence? Do you stress out when taking an exam?
Some people either worry, stress, or constantly force themselves into studying harder. Try to make studying something a little more fun! (song, poems, riddles, and etc...)
When taking an exam relax and have confidence. Don't over study because when you do, people tend to stress more about how the exam is likely to be. However, if you studied then you have not much to worry about no matter how easy or how difficult it may seem. If the exam is hard, take a moment to breathe and relax. Flash cards are also a good way to study.
Another thing is if you are making an effort to find the time to study? If not visit a library or find out about after school programs that you can attend. Not making an effort is not a good habit. We all have time for everything, just not at once. It all depends on how you manage your time wisely.