Nope, bodyweight is not related to starvation mode. You are either starving or you aren't, at any weight. It takes weeks of very large eating to get out of starvation mode. It doesn't happen with one meal, or even ten.
Yes, it is a property of wheat. It is why most people here don't eat it. Rice and starchy veg are fine. As hungry as you are now, wheat would actually be fine for you too.No one knows why Aneorexics feel like that. The only solutio is to eat more, as that improves the mental processes and gets you out of that state. You can't think your way out because it is a mental condition. You have to eat your way out.
Racing certainty. Your life and health is much more important then delaying some exams, and it should be obvious that anxiety about your exams is a primary cause. Taking your exams later or even dropping back a year doesn't matter in the slightest. Anyone can see you need an immediate long break from school and school related stress. This kind of thing (resits, dropping back a year, etc.) happens all the time and just doesn't matter in the long run - unlike your own health, which is the only real thing of value you have. I resat my own similar exams at 18, and it was much easier the second time round.