while

"A Palestinian schoolgirl who was hit by Israeli gunfire as she sat inside a United Nations school in Gaza has died of her injuries."
Isn't supposed to be " as she WAS SITTING" instead of "as she sat"?
thnx

It sounds fine.

Either construction is fine. There is no "required" form.

In this case, the past simple is most likely used because the speaker or writer saw the girl's position as a state, not an action. States are typically expressed by simple tenses in all contexts.

Simple tenses are more formal than progressive tenses, which is why newspapers prefer simple tenses. Compare:

I await your reply.
I am awaiting your reply.