Emma, what you're going through sounds absolutely terrible. This is supposed to be one of the most fun times of your life, and you're not able to enjoy it because of your anxiety. I truly empathize, and hope that you can get better. One thing I can say is this illness is NOT worth ruining your grades, social life, or high school experience over - I promise. So you have to keep fighting. I wish I knew of some concrete way to help you fix it (being an anxious person and emetophobe myself), but all I have is what is in the process of working for me, so I will share.
I'm 30, so have been dealing with this for quite some time, and missing out on things because of my fears. The funny thing about getting older is, you do start to realize that you don't have all the time in the world to do things. So while I'm still anxious and phobic, I also get angry at these feelings when they happen, and at myself for allowing them to. I get angry thinking that I can't go to a party or a bar or concert because I'm afraid someone might be sick. And the funny thing is, this anger actually helps me! When I really put in perspective what I am missing out on due to the fear, I am so angry at that thought of missing out on life that end up forcing myself to participate (even for just a little while). I'll go to a beer festival early instead of late (since people will be less likely to be trashed! lol) - but I will go. I'll go to the gym even if I'm feeling a little nauseous...but will take it easy until the anxiety subsides and I know I'm better. And you know what? Even if the worst happens and I'm exposed to a stressful situation, it's rare, not that bad, and overall I'm glad I went.
One other tactic that has been helpful...the worst case scenario. Especially in terms of the phobia, I've found that really thinking through, step by step, what you would do IF the worst happened has been extremely helpful. For example, if I'm afraid of getting sick...what has happened other times that I've actually had stomach flu or gotten sick? it has been unpleasant for sure, but I SURVIVED. I managed in the situation. I remember when I had salmonella that while I was literally in the process of getting sick, I wasn't anxious - I was actually thinking how amazing it was that our bodies could propel stuff out of us with such force lol (a little graphic but I found it amusing looking back). Thinking about these past experiences and the reality of them (vs. the fear we normally consider) has helped me calm down, even if I'm still terrified to get the stomach flu or something. If someone else gets sick? We can usually leave the situation...if not, we will cope. I was tested on this when I was driving my cousin around who had drank too much and was minutes away from being sick. While I felt close to a panic attack, when the time came where he was sick (in the car no less!), I opened the window in time, kept calm enough to continue driving safely, and handled the situation like I would any other slightly chaotic issue. I even helped clean him up.
We are often a lot better at handling these fears and emotions than we think we are - but if you look back to your experience (we've ALL been sick at some point), we've done it. You're here...you've been through worse. Keeping that firmly in your mind and trying to establish some semblance of control over the situation (if I'm feeling sick, I'll put a bucket by me/glass of water and gatorade/eat ginger candy/make sure I'm close to the bathroom, etc) helps because the issue really is a lack of control and a fear that you won't be able to manage the lack of control (which isnt' true). My phobia went haywire when I made two long distance moves in the span of two years. As I started to assert more control over both the phobia and my life in general (making a 5 year plan, figuring out what I really wanted to do career wise, etc), things got WAY better. But being prepared also helps. Today I know if I or my husband get sick, I have the supplies to take care of us and get through it. Even that little bit of control helps. Think about how you can apply that to your own life and regain control in areas that you might feel are in chaos right now.
Best of luck to you. I wish your family and school could provide more support, but unfortunately those who don't experience this rarely understand it. Take care of yourself and please take those chances to do the things you love - you WILL be ok and your future self will thank you.