Karma is just another way for a society to move past mans basic instincts, like any religion.
Probably the least destructive system, but it's more big picture than tests etc.
The idea is largely the same as for any morality based superstition, if we all behave to a social code,
we can achieve a state of heaven on earth.
So you cheating didn't really hurt anything in the grand scheme, but if everyone cheated we'd make tests pointless and isn't life just one big series of tests? wouldn't the excitement be in the highs and lows, the doing well on tests from your accumulative knowledge to the failing and reassessing your grasp of a subject.
I'm not a judgemental dude. I've made my fair share of bad decissions, but it's learning from them that makes us grow, not the getting away with it.