Hey there, we spell Jessi the same way XD cool!
I can't offer a ton of advice without knowing as much about why your parents don't approve. How long have you been out to them about this? It can be a pretty intense reveal for parents, as they tend to think they know you well enough to notice these things but they often don't know. Are you in the USA? When you are 18, legally you can do anything you want. I know it sounds like a really long time off, but now is a great time to start building your escape raft.
Do you have a job? Do you have particular interests? Can you think about scholarships- as in, if you are a person of color, if you are getting good grades for your school, some colleges offer some really awesome scholarships. If you have not considered it yet, look into things that will give you some skills in things that interest you.
I volunteered at a zoo in high school, which got me a veterinary job cleaning kennels. That job trained me how to use autoclaves, which I then went to work at a Piercing and tattoo parlor, which gave me more experience with sterilization processes, and now I work FT at a hospital sterilizing stuff! I certainly did my share of screwing up and wasting time, but when I got my act together it's taken me... about 2-3 years to save up for top surgery. The entire time I have worked my FT job I have been saving money and taking T. I'm 30 and only realized I was transgender when I was about 27! So I am kind of jealous of you for knowing your own needs well enough to already have come out.
It really sucks to have someone telling you are not trans or that you can't change now. I feel that! But- this is a great time for you to be active in setting up plans for getting what you want. Research! read about all the pros and cons, read about top surgery, read about the psychology and biology of being trans. There is TONS of info out there and you need to be pretty critical of all of it. That doesn't mean don't believe it, it means think about it and double check facts before you accept it as a fact or valid theory! Read up about how to propose a good argument and try to imagine the reasons why your parents may be against this.
Also, I am speaking from how I think my own parents saw me, and yours could be similar or very, very different. My parents thought they knew me for almost 30 years as a girl. I always felt something was off, but couldn't figure it out. They never really noticed it that much, so I think they feel bad that I was suffering and upset and they did not ever see or imagine that I was a transman. People want to think they are more perceptive than they may actually be- I was pretty good at hiding how miserable I was most of the time.
I'm wondering if there is a queer friendly and trans positive counselor or teacher you might be able to talk to? Have you talked to any of your friends about it? Would any of them be people you could trust to approach a staff at your school about it?
Please keep in mind, I live in California and most of the state is relatively trans... tolerant. I won't say friendly because some areas are pretty conservative or not so open. if you have doubts or concerns do not risk your safety and privacy! I spent most of 16-17 on the internet, learning about human psychology and things like transgender issues. It's kind of lame to be in the coocoon stage of being pre trans, but you are young enough that you can begin your plans now. It's easy to want to leap into hormones and stuff immediately but that is pretty heavy stuff. I'm still scared of injecting myself some weeks, even as eager as I was to see results. Sometimes good things require a lot of patience and I guarantee if you can bust butt now, you will see progress sooner. Do you ahve your own bank account? It might be good to start saving, you can set up accounts that no one can get into except you at 18 years old or even 21 years old. If you chuck any allowance or odd change you have into it every week/month you can start saving now. Surgery is expensive and even if you get a job that will pay for it, having a fat wad of money to keep you fed and your bills paid while you are waiting around healing is SO GOOD to have!
