Praise be to Allaah.
Firstly:
It is obligatory for a girl who has reached puberty to cover
her entire body in front of non-mahram men, because of evidence that has
been discussed previously -- see questions no.
11774 and
21134.
There is no differentiation in this regard between the
street, the school or any other place. Wherever there are non-mahram men,
the woman is obliged to cover all of her body, including the face.
Secondly:
You have to try hard to convince your parents to let you
study in a school that is not mixed, or a school in which you will be able
to cover your face, or to study by distance learning, so that you can avoid
falling into sin.
If your family do not let you do that, and they insist on
your staying in this school, and you cannot move to another school, then you
have to wear the niqab outside school, because that is what you can do. The
fact that you are forced to uncover your face in the school does not make it
permissible for you to do so outside of school, because Allah says
(interpretation of the meaning): “So keep your duty to Allaah and fear
Him as much as you can” [al-Taghaabun 64:16].
We ask Allah, may He be exalted, to increase you in knowledge
and guidance, and to grant you a way out and relief.
And Allah knows best.