Probably a stupid question but about phones?

I have an iPhone 6s, and I want to buy and iPhone 8 Plus, which I want to buy the cheapest option (so there be little 4g) and use my current SIM card in the 8 Plus and put the 8 Plus SIM card in the 6s and give it to my younger brother. I'm sure I can do this but I just wanted to make sure.

How do I buy a smartphone?

Sounds like a dumb question, but hear me out. I'm 17, and ever since I got a job at a fast food joint I've been wanting to get myself a new phone with my own money. I currently own a cheap $50 Samsung that my mom bought for me, and I'm part of a family plan (Metro PCS) where the four members of my family pay a total of $100 for 4gb of data each, per month.
My question is if I wanted to buy myself a more expensive phone ($200 - $300), how would I go about doing this? Does the phone HAVE to be one of the ones Metro PCs has on display? What if I wanted to buy a used iPhone from somewhere else? Can I do that? How? Any info will be appreciated

My parents want me to switch schools?

Currently I go to a school hat has a messed up grading system therefore when I should be getting in the high 90's I'm not. We've contacted my school and they said they can't do anything about it. So my parents want me to switch to a different school with a public grading system. They said if I do, I'll get the newest iphone. But just this year I finally feel as if my life has finally come together as all the bullying stopped, I dropped my toxic friends and I found a new group of people who really care about me. What do I do?

How to turn off AT&T smart limits parental controls on an IPhone without having to log into the account?

Okay so about a year and half ago when I got my first phone my dad put this Smart Limits thing on it when him and my mom we're both on our account. The problem is about 2 months ago he left and hasn't come back. Nothing new, but when he left he had these parental controls on my phone. (My data doesn't work, I can't text certain people, etc). The problem is that I really need access to everything and everyone he blocked. I realize that you have to log into this service but I was wondering if there was a way I could hack around it or if I would just have to call at&t and explain my situation. I'm certainly not calling him so that isn't an option.