Hey, I have acid reflux and have since I was your age.
Okay, you'll have to avoid spicy and acidic foods as much as possible. Orange juice, tomatoes, chili, things with a lot of onions, all of it. If you must eat it, eat as far from when you'll be sleeping as possible. And drink tons of water. Mine also seems to get worse when I haven't eaten enough, so make sure to eat breakfast.
Sleep on your left side, for some reason that seems to keep the acid where it belongs. I dunno why, but it works. Also sleep on a slight incline, either with pillows or (preferably) put the top two legs of your bed up on some blocks of wood so the whole thing tilts. It takes a little getting used to to sleep like that, but it works as well.
You already know this, but try to stay calm. Do stress-relieving activities. Take a nice bath, read a good book, play video games (I love Animal Crossing for this). Whatever works for you. Stress will always make acid reflux worse.
Calcium will help. Tums are basically calcium tablets, they use them as supplements sometimes. You really shouldn't live on those- for a long time I went through a bottle a month. Sometimes a doctor can prescribe something to help, but it didn't work for me. It might for you. In a pinch, eat a bit of ice cream, or a piece of cheese, or drink some milk (if you're not lactose intolerant). It's a temporary solution, but it'll help when you're desperate. I also always keep water near me, and drink a few gulps when it starts getting bad.
If OTC stuff doesn't work, seriously considering asking your doctor if they can prescribe something. A lot of meds don't work for me, so they probably will for you. It'll keep you healthy and really improve your life. And focus on stress-relief, especially during exams! Take some time for self-care, whatever that entails for you. It's really, really important for anxious people like us.