The cynical adult in me (who has 3 kids, a full time job and studying full time) had quite the chuckle at the Junior High responsibilities and lack of time.
I'll give you the bad news first: It doesn't get better. There's the old adage that you can have money, time or energy, but not all three.
Good news is, however, that you are young enough to get into good habits that will set you up to manage.
As Dale said, prioritization is key. As is looking at what's really important, and identifying what's a want as opposed to a need.
It does take a lot of self discipline, however, and it may mean sacrificing something - either social time, personal time etc.
But as you mention you're studying - use writing as a reward. Organise your study in blocks, don't try to do too much for too long as that can actually have a detrimental effect, both on you and your studies.
There are numerous reports and studies done about effective learning, and a general consensus is to do no more than an hour straight in a single session. So do that. For every hour of study, give yourself a 10-20 minute downtime to write or otherwise create.
And do not overlook the benefit of exercise. I'm not talking about full gym sessions - but get out in the fresh air. Start the day with a run or walk. Take constant breaks and stretch out. Get up and walk around. Keeping active and healthy also helps you mentally as well as physically.
If you keep getting ideas, carry a note-book or other app (i.e. Writer+) or something with you to jot the idea down. Then go back and review later when it's your time to write.
It all boils down to structure and discipline. Structure your day, have the discipline to stick to it and get into routine.