Hi Ozan,

The expression être parti/e  (to be gone) is a passive form of partir so it is a present and not a past.

I would translate the sentence as -

She has to be gone before midday = Il faut qu'elle soit partie avant midi 

(subjunctive present of être)

different from the active form of the verb, to go which is partir.

She has to go = Il faut qu'elle parte ( subjunctive present of partir)

Bonne Continuation!