What

The girl who always looked back have an 'illness' that makes her turn backwards.

Infact it was just her previous turning 'leftover' (namely the nose) that makes her think that someone is behind her and she imagined all short of things.

Based on everyone behavior in school this has happened often and she always forgot that it happened.

The last few pages is just a new 'turning' that no one expected.

Ah that's why the people not panic even her friends also looks nonchalant about it