I have to agree with Gok Wan on here,
You couldn't eat for a week, and go without absolutely anything. Your body would go into shut down mode and stop working.
You'd find it impossible at school. Tired, hungry, unable to concentrate, you'd be too tired to do anything. And probably wouldn't sleep very well at night because all you would hear and remember would be your stomach rumbling and how hungry you were.
And even after a week of this, yes, you may have lost about 5 pounds. BUT, you would put it on straight away, and you'd never really notice that you'd even lost anything.
Please, if you don't take it from any of these, take it from me.
(An 18 year old, recovered anorexic from the midlands.)
Because I know exactly what it's like.