We are our own biggest supporters.
We believe that our strengths are unique and that our failures are either 1) not our own fault or 2) extremely common. Your classmate's high grades aren't that impressive - you could easily match those if you wanted to. Who cares if you don't get much done at work? Not like anyone else is putting up extra effort - and even if they did, they probably don't have as much going on as you do.
We fall victim to self-serving bias all the time. An interesting experiment my psych prof once recommended: take out a notepad and jot down every time you make an excuse for a failure. You'll be amazed at how long the list will get.
And that's exactly why self-serving bias not a problem: our self-esteem would be battered without that spiffy little mechanism. High self-esteem makes us think we can be great things. High self-esteem makes us think we can achieve big things. And if we believe we can do it, our chances of succeeding actually increase.
The key to balancing is to be our own biggest critic.
After all, criticism is easier to swallow when it comes from yourself.