It is late on a Monday afternoon and Frederick A. Birkett, head of the Benjamin Banneker School in North Cambridge, is walking through the school parking lot saying goodbye to a teacher.

After he bids the teacher farewell, he calls over to sixth-grader Genevah G. J. Alphonse Dalina, who is watching a low-key pick-up basketball game.

Lost in the game, she doesn't respond. So Birkett playfully remarks that he is not talking to the fence.

It might be a typical scene from a typical school, but Birkett--dressed in a black-checked bow tie and coordinating jacket--oversees Cambridge's only charter school.

Which changes the interpretation of the scene.

Consider:

Dalina attends the school because she was selected in a lottery. The school.is 97 percent minority, by far the highest in Cambridge. The teacher Birkett was talking to has no tenure--like all Banneker teachers, he's on a one-year contract. Dalina was still wearing her school uniform. The basketball players--shooting hoops nearly two hours after school let out--were a student and a teacher playing one-on-one.

And Birkett? He's not a principal, but an executive director.