"I am not at the house, but at the school."
Translation:Nem a háznál vagyok, hanem az iskolánál.
I think the English translation better corresponds to the inessive "nem a házban vagyok, hanem az iskolában". To my ear if you are "at school" you are in the building. If the emphasis here was to say effectively "I am not located adjacent to the house but adjacent to the school" i.e. waiting or standing next to the locations in question, a better although oddly specific way of saying it in English would be "I'm not by the house, but by the school".