For some students, success in school means getting A's and B's. For others, it means getting straight A's, starring in the school play, and winning the class election. Success means achieving a goal, and everyone's goals are different.
We tend to think of success as a triumph or victory, but if you look at its linguistic roots in Latin, success literally just means "result." At some point several centuries ago people probably began using success as shorthand for "good success," and eventually they dropped the good altogether. That would explain why in formal settings you still occasionally hear the phrase "good success," even though we now think of all successes as good.