Dropping out: Information from Answers.com

Dropping out means leaving a group for either practical reasons, necessities or disillusionment with the system from which the individual in question leaves. It is used in various contexts, including:

Most commonly, it refers to a student quitting school before he or she graduates. It cannot always be ascertained that a student has dropped out, as he or she may stop attending without terminating enrollment. Reasons are varied and may include: to find work, avoid bullying, family emergency, poor grades, unexpected pregnancy, bad environment, lack of freedom, and boredom from lack of lessons relevant to the world of work. The Silent Epidemic: Perspectives of High School Dropouts^[1] by Civic Enterprises explores reasons students leave school without graduating.
In the 1960s, "dropping out" was used to mean withdrawing from established society, especially because of disillusion with conventional values. It is a term commonly associated with the 1960s counterculture and with hippies and communes. See Turn on, tune in, drop out.
The academic, Robin Farquharson, wrote a book; entitled Drop Out!, about his own experiences dropping out of university life after he saw Timothy Leary's "Turn on..." statement on television.
In clinical trials, participants may withdraw from the study, for example, due to adverse effects. This is also referred to as dropping out.