Freshman can be seen as "fresh to high school." High school is often the place of increasing maturity and global acceptance. From middle school you have numerous cliques and groups, with thousands of "rules and regulations" you must follow in order to be accepted. In high school, you can remain your own person and become accepted by at least several others. As such, these 9th graders just came from the depths of middle school, so they're fresh into the system of acceptance.
Sophomores already have a year of experience in high school and they know how well they can fit in. However, they take advantage of being able to easily fit in, or they attempt to redeem themselves easily. Sophomoric is a term that means immature... we can see sophomores as well-accustomed but still not completely matured yet.
Juniors are usually signified as those who learn under others. They realize what's right and what's wrong and act upon it. They begin learning from people instead of simply copying them, and they begin to phase out of the immature stage.
Seniors have accomplished acceptance and maturity. They are ready to graduate into the real world.
These are just my own random definitions. Not everyone is mature as a senior, and not everyone is accepted. Not everyone is immature as a sophomore or freshman, and not everyone is rejected.