struggling to concentrate

I think you should really focus on your classwork at the moments and try to finish as much as you work as possible during the school period time. After school, you definitely catch up with your work from the beginning. It's important that you the base for other information to add onto. I would say watching YouTube videos like Freesciencelessons for science will really help. Or use websites such as Kahn Academy or Physics and Maths Tutor. I was actually like that once but I got out of that situation by talking to the year leader and motivating myself as much as possible. I'm not really in a position to give tips since I'm in year 10 but just do your best, and that will be enough. You'll see that slowly you'll start to catch up and whenever it's your holidays or werekends, make sure to do understand that content first before rushing it down on your book or paper.