I'm like an hour into the game and i'm not seeing the criteria for killing challenges (rather than just owning them).  Is there a way to? 
 
Also, are there skate.school challenges after the park creation stuff?  Am I missing something for the Coach Frank skate school achievement? 
 
edit: OH okay, the challenges just fade the kill/own in and out, nevermind that. 
 
can you not kill every challenge, though?

some challenges such as own the lots, contests, 1ups don't have a kill it options.  the kill 80 challenges achievement basically requires you to kill every killable challenge.  

but maybe not because it shows that I only killed 75 challenges and got it.   i'm not really sure whether its a glitch or what but it's very nice because a couple of the hall of meat kill its are harsh.