There are certain events that are really important, but you can't expect people to remember the date of every single important event of WWII, or of every war for a matter of fact. Don't get upset if your classmates can't date the start of WWII. Without looking it up, can you tell me the start date of The Revolutionary War, the Civil War, WWI, Vietnam, Korea, First Gulf, Afghanistan, Iraq? If you want, use the opportunity to educate and give perspective, but don't get on a high horse because you know the date.
I don't think there is a reason to be depressed on December 7th. To me, it's a reminder of the service men and women of WWII. The glass is half full.