I had actually been out this way before to use the community centre, but at the time I didn't realize that Templeton was the school that had stood in for Truman High in After School Special (4x13). Here it is in all its ordinary glory:
The front of the school itself isn't actually filmed that much in episode. What I found most recognizable was actually the street leading up to the school - remove the parked cars and picture the Impala pulling up among fallen leaves *rock music plays* and there's your iconic flashback opener in the episode:
I could have wondered over to the baseball diamond (where I'm pretty sure Dean threatened to "tear that guy's lungs out!") and taken another picture, but it IS actually a school day and I didn't want to look too much like a creepy stalker or something...plus, you know...crochet hooks were waiting to be bought.
Hope you enjoyed your brief look at a Supernatural location...
I should point out that just a few blocks further than the yarn store are Rufus' and Chuck's houses. It's 2.7 kms from my place to the yarn store though, and I didn't really feel like walking further - plus, those are private residences, so taking pictures of them would be a bit creepy.