I would first suggest that early adolescence might be the easiest stage to focus on in this question, as this phase is most often defined by the onset of puberty and the physical and emotional changes that come with the hormonal fluctuations. Middle and late adolescence are more typically defined by statistics and average ages, and are slightly less distinct in their qualities and characteristics than early adolescence.
The most important characteristic of identity development during all of adolescence, but especially early adolescence, is that this is one of the first times in a human's life that major physical, psychological, and therefore, emotional, changes take place. As a result, most experts agree that identity development during this time is often full of insecurity, confusion, and peer-comparison. As a result, most adolescents learn best by trial and error. In early adolescence, when hormonal changes begin at puberty, identity development depends on a balance between clear boundaries, emotionally secure relationships, and positive older role models.
Peer pressure becomes more influential during adolescence and begins in early adolescence. At this time, identity begins to be shaped by who students hang out with, who accepts them, and who rejects them, socially. This is why early adolescence is the best time for parents, care takers, teachers, and/or coaches to secure the balance mentioned above. Because students are still young enough and the psychological and physical changes are new enough, they are not as likely to rebel in early adolescence. Experts also agree that when students have positive relationships with adult role models, they are far less likely to be persuaded by negative peer pressure (and fall into substance abuse, depression, failure in school, etc). These relationships are equally important at home as they are at school.
One final noteworthy characteristic of identity development in adolescence is that it is another jump in a human's exploration of independence. Consider the differences from an elementary school classroom (and schedule) to a middle school classroom (and schedule). Students are expected to do more work on their own, to be more responsible for their personal belongings, even to get themselves to each class on time through the ringing of bells. For most, early adolescence is a time of adapting to these work changes and expectations. Many public school teachers, administrators, and parents, have debated the jump in independence (and expectation) from 5th grade to 6th grade. Some advocate for a more lenient transition, worrying that too many middle schoolers "get lost," socially as well as academically. Others have suggested that middle schools in general are already too lenient and most students are entering the 9th grade, unprepared for high school level work.
All of this is to say that the time period in a human's life labeled "adolescence" is very broadly defined in terms of age, behavior, identity development, and social or academic expectations. Ideally, students could be worked with in smaller groups, where adults could teach and reach students on a more personal basis.