When DS refused to eat, I used to make very high calorie cakes full of protein because even if he gave up after a couple of bites they gave so much goodness in a very small amount of food intake. Use almond and soya flour instead of wheat flour, add loads of eggs, make frosting with cream cheese, vanilla and icing sugar. Good ones include: carrot cake with walnuts; date and walnut cake; banana bread with cream cheese topping; chocolate and hazelnut brownies; fruit and nut flapjacks. If they're not allowed nuts at school just go for soya flour, plenty of eggs in the cake mix, cream cheese topping and as many dried fruits as she'll tolerate in the recipe.
A good slice of this and a fruit smoothie or milkshake will give her about 500 calories. Even a small slice and a drink will give her about 300 which is better than nothing.
Or she could have a small slice of quiche or a mini quiche with some raw veg sticks. Again, the combination of pastry, egg and cheese packs in a lot of calories in a few tiny bites.
Are you sure she's not purposefully keeping herself thin due to peer pressure?
If you can ensure she has protein and some slow release carbs at breakfast (eg porridge and a homemade banana milkshake or wholemeal toast with peanut butter, cheese, eggs or beans and a very healthy evening meal, then she should be OK.
Are they allowed to suck mints or throat soothers during exams? If so, the sugar from those will be enough to keep her brain alert during the actual exam sessions and as long as her other meals are really nutritious, it shouldn't do her any harm.