Is there a possibility that you misheard your classmate? Maybe what he actually said was something along the lines of
Der Platz ist schon frei, oder? [Emphasis on "schon", rather than "frei".]
This is a very common (if colloquial) use of "schon" as a particle in questions and has nothing to with the meaning "already". It expresses that the asker is pretty sure that his assumption is correct. An English equivalent would be
This seat isn't taken, is it? [With a heavy emphasis on the "isn't".]
Canoo has a short entry on "Abtönungspartikeln", aka "Würzwörter" or "Modalpartikeln". These little words allow the speaker to express expectation or his attitude. "Würzwörter" is an excellent name for them, because they are often what makes a sentence idiomatic, like the pinch of salt that rounds off a dish.
However, like with salt, it's sometimes difficult to find the right dosage: non-native speakers sometimes try too hard and sprinkle their sentences with them too freely. A very dear American friend of mine speaks almost flawless German - but her overuse of "ja" in virtually every sentence gives her away. Unfortunately, it is almost impossible to really get the usage of these particles right for non-natives.
Note: The Wikipedia article on Modalpartikeln is not complete. It fails to mention this rather colloquial meaning of "schon"; it only lists:
"schon -- Negierung der eigenen Wichtigkeit an einer Sache: „Was kann
ich da schon ausrichten?“
Also, have a look at what Emanuel writes in his blog -- as far as I can see, he covers pretty much every instance of "schon" :)