Will I lose weight?

So I've posted here before, but things are somewhat different now.

Only a week ago, I would have anwhere from 400-850 calories a day.
After seeing what just happened to my sister (she got put into some sort of hospital for anorexia), I decided I really shouldn't ruin myself too.

Now, I have from 1200 to 1300 calories a day.
But it's only been 3 or 4 days I've done this.

When I did the 400-850 thing, I weighed 134, sometimes even 133.5 or so pounds and it was great.
Now that I'm eating more, I'm back to 137-140.
I'm pretty sure I've been that weight, only, I didn't have enough food in me and stuff.
I didn't gain weight in fat, right?

So my question is: If I continue the 1200-calorie diet, am I guaranteed to lose weight?
I'll eat anything and I don't eat anything that doesn't have a lot of protein or fiber and such in it.

Oh, and another question..
Peanut butter is definitely a weakness of mine.
I guesss I don't eat it in moderation, but I count it towards my calories.
Will it make a difference if I cut down on it?

And I forgot to mention..
I'm a student [[high school]] and I have a really tough schedule this semester so I won't be able to work out [[on the things I have here at home]] as much. So the only time I'll really be getting a lot of exercise is on the weekends and during gym.

Also..does anyone suggest anything for my situation?

EDIT:
Sorry, wow, yes, that would help haha.
Age: 15 (16 in June), Height: 5"2'
And on Friday I woke up and I was 137, today I woke up and I was 140. =/
I'm afraid to even check tomorrow. I 'll try not to.

What are your stats?  Age, gender, height and weight (138.5?).

It is hard to give advice on what to do when we don't know much about you.

Why I am asking is that first off, you need to make sure that the 1200 is the right amount for you.  Many people go from really too low calories to the 1200 mark simply because its the bare minimum that your body needs.  If they should be at 1700 and eat 1200, then they are in the same sinking boat.  Only difference is that they have a bucket to bail out some of the water while the boat is going down.

You were starving your body.  You were hurting yourself when eating so low.  When you start to eat normally again, it will take some time to convince your body that this is a permanent change.  It takes time for it to feel comfortable and to start letting go of the weight.  This does involve a temporary weight gain.  But if you continue with a healthy eating patern, this weight should come off.   The longer and harder you were hurting your body, the longer it will take for it to come back to normal.

Its ok to eat some low protein, low fiber things.  If you try and limit yourself, you will not be able to stick with this.  Work in some small treats during the week.  If you don't look at this as a healthy life style change, you will end up depriving your body and your mind and you will probably end up posting here about the 4000 calorie binge that you had.

Make sure that YOU can live with this change for the rest of YOUR life.  Not just for the next 5 months or 2 years.

{EDIT}  I just re-read your post and noticed you were in high school.  So you are what, 15-17?  At this age, you should really not be into counting calories too much.  Your body is growing and changing all the time.  You need to keep it nourished.  The regular rules that apply for calorie counting and how bodies work do NOT all apply the same way for kids.

Make sure you are followed by your doctor on this to make sure you don't do any more harm to your body.

I think the question here is "how to I stay at a healthy weight?"

Seeing as you have no profile so we don't know how tall you are or what your build is.   The only thing we can go on, is that you are a teenager and still growing.

What many doctors and nutritionists and many members of this site suggest is that when you are a teenager, you should not restrict your calories unless you are extremely overweight. 

Concentrate on exercise, even though you may not have alot of time.  Try to join an extracurricular activity at your school that gives you good cardiovascular exercise like soccer, football, basketball or track and field.  Even cheerleading can be good exercise.

You should also focus on eating a healthy diet full of vegetables, fruit, lean meats, oils and healthy dairy.   Eating less fatty and processed foods will help you reduce your fat even though you may be eating the same amount of calories.   Your body will have more energy and you will be naturally more active.

Don't worry about those extra pounds or looking like a model or an actress,  these people are sick.  Just look at what happened to Anna Nicole Smith.

The most important thing is to concentrate on eating well and staying active and as you grow older, you will stay youthful and healthy while others who may look great now, will not.