In considering an 80-minute play with 20 characters, one expect to see a considerable number of cast members on stage. Nilaja Sun’s No Child. breaks down this assumption, as Sun is the one and only cast member in the production, which is being performed at the Loeb Drama Center through Dec. 23.

Sun’s ability to convincingly switch from character to character is remarkable; she’s consistently accurate in channeling different personalities. Thanks to this ability it’s never confusing as to which character she is portraying.

No Child. is based on Sun’s experiences as a drama teacher in New York City’s inner-city schools, where her students were usually either physically, emotionally or sexually abused. The play, based on Sun’s actual experiences in the public school system, follows Sun into the classroom where she has six weeks to teach the class a play to perform at the end of the school year.

The American Repertory Theater’s set for No Child… is minimal, with one back wall containing two beat-up doors. Stacked against it are rows of chairs, which create the illusion that there should be more kids in the school than there are. At center stage there is a row of three blue chairs equally spaced on a tiled floor that represents a high school classroom setting.

The significantly small set design alludes to the poor conditions in these schools, and allows attention to remain on Sun at all times.

Sun introdcues the play in the guise of the school’s hanitor, who is the narrator and comic relief. Having seen many people pass through the school, he helps establish the setting. After this introduction, the play delves into the school atmosphere, as Sun enters the classroom ready to teach drama. The play chosen, Our Country’s Good by Timberlake Wertenbaker, later helps propel the story line forward. Its tale of convicted felons aids our understanding of the classroom environment since we learn that the kids know that people expect them to end up in jail, drop out of school and get low paying jobs.

Through the perspective of the teacher, Sun sets up the picture of an inner-city school perfectly. She explains how most of the kids are always late and that they are very “spirited students.”

In fact, we see many instructors go through the classroom with Sun being the only constant factor besides the kids. Being able to see all the teachers come and go shows how even if the kids had wanted to learn, it was almost impossible because the teachers were usually petrified and exasperated with trying to get them to focus that they just gave up.

Sun’s portrayal of each student is dead-on and she effortlessly evokes the ambiance of the classroom. Her characterization of the students and teachers allows us to become attached to each of them and feel their emotions.

Sun’s ability to integrate the real atmosphere of these public schools isn’t overwhelming. At no point does the play feel like she is trying to brainwash us into making a difference. Instead, Sun leaves it up to us to decide what our part should be in changing the education system.

Her knack for including references to pop culture-at one point a rap from one of the latest Kanye West songs and referring to other well-known subjects-keeps the audience laughing, attentive and truly interested in what is going on before them.

Sun was also able to switch from a humorous aspect in the play to a serious one in the blink of an eye. Although there’s much to absorb from No Child…, Sun’s skill makes the experience of several days in the classroom go by quickly and the dense information about the public school system easy to understand.

Sun ends the play with the performance of Our Country’s Good. Even though crucial lines ended up being fumbled and some parts are hard to understand, the students pull off the performance. Some enjoyed the experience so much that they even joke that they will get left back just to have her as a teacher again. The ending leaves the audience thinking about the public education system in place today. Now that’s a lesson.

No Child. performances are at the Loeb Drama Center at the American Repertory Theater in Harvard Sq., Cambridge until Dec. 23. Student tickets are $25 dollars and student rush tickets at $15. Times vary. For more information, call 617-547-8300.