I've seen a lot of discussion and argument about the terms "old school" and "new school" and exactly what they mean. Here is the best way to describe old school play vs new school play I've found so far. It's far from prefect, but it seems to get the major ideas across to those I talk to better than anything else I've tried.
Let's say there are two major styles of role playing games. From a player point-of-view, the first (and older) style says "Here is the situation. Pretend you are there as your character, what do you want to do?" This style has been superseded over the years with a style that says "Here is the situation. Based on your character's stats, abilities, skills, etc. as listed on his character sheet and your knowledge of the (often many and detailed) rules of the game, what is the best way to use the game mechanics to solve the situation?" Old school play strongly favors the first style and frowns on too much of the second. If your game tends more to the first than to the second, it's leaning "old school." If your game tends more to the second than the first, it's leaning "new school".