Free time depends on a school you're attending, on teachers and lections. I attend a really hard school and the teachers are very strict and non-human. So, usually, I have a small amount of free time and I consciously use it wrong (checking my social media and stuff). If you have the same problem but you want to have good grades and school is important to you but as much as your social life, here are tips for you (you actually need to try them to see that they work and to get used to):
As soon as you get home from school, do your homework. I come home from school usually around 1pm and I eat lunch with my family around 3pm. So, when I come home, if I don't take my phone, I'm still concentrated and if I sit at the desk or table, I can do my homework really quick so I have whole afternoon to learn other things. Another good thing about that is, for example, if I'm doing my math homework every day, I don't even have to study for the test and I have more time again.
Be active in school so teachers can give you better grades. Again, this depends on school. In my school, we have like verbal exam idk, so if you're active on every or almost every class, teacher wont even give you that exam. If your school have something like that too, try that because, again, then you have more free time.
Make sure that everything you're learning is important. Be careful when taking notes or highlighting because more unimportant informations you have, less free time.
Learn only after you understand what you're learning. Don't try to remember something that you have no idea what means, you will lose tone of a time.
When you have free time don't check your social medias. You can do that in the evening before sleep, I swear you won't miss anything. After you finish one subject, start with another or at least plan what you'll do next, drink water, eat something but DON'T take your phone, you'll use it much more then you expected and after all that you won't even go back to studying. Also, you can go for a run or excersize because that will boost your mind and memory. You should have 15 min breaks.