Balls off the fence

[fguyton]

At my son's game last night (12 year olds / travel) an opposing player hit a fly ball that hit the fence in center field near the top, bounced up in the air and then was caught by the Center Fielder. I "knew" it was in play and the umpired ruled it as such... but the coach argued that it was an out. I told the coach I agreed with the ump (I'm the score keeper) and he asked me to show him the rule. Well I had my NFHS and MLB rules with me but could not find it.

I know the fence is treated just like the ground, i.e., in fair territory, but can someone point me to the rule?

thanks,