That's an excellent way to get rid of all the muscle.
Your body gets energy from the fat cells, once there's no more, your body gets energy from your muscles.

dont do 300kcal a day. i lost weight that way BUT got an eating disorder trust me its not fun. the shitty part is the second ull start eating normally, ul gain the weight. best diet: 1000 kcal a day nd exercise about an 1hour 4-5 times a week.
u didnt get this weight in a week, and itll take more than a week to loose it.

27.4 BMI? That's not too bad considering you not in the obese range...

I can't believe your parents are like that towards you that is so harsh you should tell them how you feel.
Yea 300 calories a day is not where near enough, your body will just shut down if you do this for long enough and that's obviously not good.
Just cut out everything that is junk in your diet like chocolate and stuff like that and focus on eating fuit & veg and healthy things, that's what i did when i started to put on too much weight. Also exercising is always good but over working your heart will put too much strain on it so you have to be careful.

eating less will make u gain more weight. Your body will think that you are trapped in a cave somewhere, so it will start to convert all the food u eat into fat AKA "hunger mode". In return you will get fatter around the stomach and chest. Your face will go pale, arms will sag and you will feel very tired, like you are actually trapped in a cave.

Is it even possible to constantly survive off of 300Kcals a day?

i haven't not read all the comments on this, however, your not fat or anything. I lost weight by actually pushing shopping carts, being on my feet all day running around at the food store IGA. I've lost atleast 15 pounds in like 5 months (which in my book isn't much for me). But at any rate, you look fine. I wouldn't worry about it that much right know, but i would remain at the current rate your at so personally id just eat the way you do know but however just like stop eating certain things. If your just snacking here and there don't do that cause i used to that and lost 3 pounds from that. So I dont know if thats the best help in the book, but i certainly did my best.

Don't eat 300 cals a day.
When you're body runs out of fat to eat, it eats muscle, and it turns horrible when you get made fun of because you can't run a lap in PE.

Anyway, don't lose weight?
Judging from you're avatar, you quiet nice, I'm turned on :o