make it clear to yourself that doing no work now will make your life very stressful and hard when a levels arrive. therefore doing 30-60 mins of each subject per day going over what you're learning at the moment will help a lot for little effort.
its good to have motivation, however when motivation isnt there you always need the discipline. so get into a routine of doing this during free periods or after school. even if you're not motivated, once you start you will continue to do it. its literally 3h out of the 16h that you're awake a day. do it to improve your grades and life in the near and further future.