[quote=@BrokenPromise]
Likely because...

#1: Your body has become so use to being caffeinated that one cup doesn't bring on the "boost" anymore.

#2: It was a weak cup of Joe. (too much water, too few beans, etc)

#3: IT WAS DECAF! 
[/quote]

Maybe school coffee just sucks... It definitely wasn't decaf, though.