Disparities in school discipline

The Wash. Post reports:

Across the Washington area, black students are suspended and expelled two to five times as often as white students, creating disparities in discipline that experts say reflect a growing national problem. ...

Experts say disparities appear to have complex causes. A disproportionate number of black students live below the poverty line or with a single parent, factors that affect disciplinary patterns. But experts say those factors do not fully explain racial differences in suspensions. Other contributing factors could include unintended bias, unequal access to highly effective teachers and differences in school leadership styles.

Are those really the only explanations that the experts can imagine? They ought to at least consider IQ, future time orientation, ADD, and drug use. And whether the students are discipline problems in other settings.