Often the lack of Iron in a teenage diet is a cause of lack of concentration and tiredness; Mild Anaemia.
I recommend you get your bloods tested and if it is observed that you have a lack of Iron, please take Iron Supplements, and go outside in the sun more in order for you to get Vit D, aiding with the intake of Iron.
Before doing this you should ensure that you get atleast 8/9 hours sleep, preferably 10 hours and you don't have any phone or devices nearby when you study and when you sleep.
Also make sure that you have a fixed sleep schedule and that you actually stick to it on the weekends.
I know that it's hard to resist staying up late on weekends but I promise that it'll help you wake up and fall asleep more easily on weekdays.
What I like to do is just go to sleep at the same time everyday, but on weekends I just turn off my alarm to get a little bit more rest.
Also , once you really get used to a sleep schedule, you won't even need an alarm; I tend to always wake up right before my alarm goes off when I'm deep into a school term
The pharmaton tablets are brilliant. You have to take one every day and not forget. They can take about 5 days to start noticeably working but I've been taking them since we went back to school and I feel great all the time. They probably help the immune system as well because a load of people in my school are sick atm and I usually catch it but Ive been grand.
Get a regular sleep schedule and don't sleep really late at weekends because that throws everything out of joint. Drink water constantly to keep hydrated. During every 30 minute study session I do I drink a glass of water and I drink minimum 1 litre of water during school and more when I come home and its great as well.
May the odds be ever in your favour!