Is the school mad. There must be a number of parents who think this is mad. I'm a Christian and we believe in fasting but not for children.
On the other hand if all the children are doing it will she feel left out if she's not aloud too.
Maybe calling school 1st thing for them to explain is a good idea. If she real wants to do it can you not get her some kind of energy sweets and energy drink so she keeps her strength up. But I agree personally that I wouldn't want my dd to do it but also wouldn't want her to be the only different one in case the other children picked on her.