Good on your for squatting 3x a week at 17, that’s going to build a solid base that will pay off in your 20’s like you can’t believe.
If your butt is rising up on the squat it’s due to quad strength. Basically as you go below parallel that’s where the most quad demand takes place, if your quads are weak, then your body is going to rely on your glutes and low back try to get it, this causes the forward lean.
What you need to change right away for the next 6 weeks is to FRONT SQUAT 3x a week, trust me, this one thing will build your squat form like you can’t believe. it’s going to be exactly what you need to coach yourself on an upright posture which will translate very well to your back squat.
google russian squat routine, it’s a really good one.
Also, what will help immediately is plates under the heels, use 25’s, no joke. this is going to allow your knees to travel way furrther forward allowing you to stay more upright and put more emphasis on the proper squatting muscles of the quads and glutes. Or do one better and get weightlifting shoes.
On that same topic, starting an immediate stretching and soft tissue program for your calves and ankles is going to be a great help. This is as easy as foam rolling your calves, and stacking one leg on top of the other for added pressure, as well as getting a thera stick to roll out your calves by hand, these work really well for this. Also, simple straight leg and bent leg calf stretches work great. another way is to stack 2 25’s and do squats with your knees coming way way forward for a warm up.
From there make sure you stretch your hips, hamstrings, lats and pecs before squatting, loosening up those key areas realy allows you to get into proper form.
That’s about it, good luck man, and post a video