"The school is opposite the restaurant."
Translation:Tá an scoil os comhair na bialainne.
Yes, scoile is the genitive of scoil. And because scoil is feminine, na scoile is the genitive of an scoil.
Most "compound prepositions" like os comhair take a genitive object, therefore na bialainne.
But "the school" is not the object of a compound prepositions, it's the subject of the verb, and remains in the nominative.