C Lestin Q.
As a student who is currently multiple extra-curriculars, I can relate to this question. For me, I workout earlier in the morning, this gives me energy to start my day. If you can't wake up earlier maybe you could try to get to bed earlier by ignoring distractions like devices. Exercise comes in many forms; if you don't have time to workout in the morning, running/walking to school could be a great alternative. This is just what I experience during my every day life, I understand that others may be busier with school work.
I advice you to take short breaks while doing your homework. Otherwise yoy strain your eyes and get bored. During those short breaks, pick up sone small exercises you can do to keep fit.
Wake up early in the morning to except use. It will help wake up ur body and it will help u be a little more energized in the mornings. It is also good in general to work out in the morning.