The best time to study is after taking proper rest. After your school take a proper rest of at least 1 hour then you can study as this helps in building a good concentration level. If you go to tuitions, like in the afternoon then after coming from there take a rest of half an hour and then start with your studies. If it is your holidays then prefer studying early in the morning, around 6A.M. as at that time one is able to learn quickly as he is with a fresh mind. It is necessary to study after taking rest as if you'll not do so you'll not able to concentrate and you'll feel sleepy.