Lockers are school property and are subject to search at any time with or without prior notice, or stated or unstated reason.
Trust me fellas, I know all this. Last year 12 kids from my grade (out of 50, mind you, a big part) were suspended because of things found in their backpacks. The school was doing searches of lockers and backpacks. Eventually all but 4 of them were unsuspended and apologized to because evidence for their suspension was found in their backpack, which is personal and private property, as opposed to their locker. The other four stayed suspended because they were retards and were smoking weed in the halls after school the next day (fucking morons).