To test the effects of black English on school performance, 27 second graders in a Harlem school were interviewed and tested for competence in spontaneous speech, comprehension of oral and written materials, and oral reading, as well as for explicit grammatical knowledge of standard English morphemes often missing in black English. The data show large individual differences in ability to use standard forms and low but significant correlations between speech and reading performance. Instruction in use of standard forms significantly influenced several performances. The speech patterns and grammatical knowledge of the Harlem children were compared with those of second graders in a predominantly white middle-class school in Connecticut. High school students in Connecticut were also tested for their use and comprehension of standard verb inflections and negatives. Data from developmental English classes showed white students better able than blacks to do grammatical exercises involving standard English. An attempt to improve use of standard verb inflections and negatives through systematic instruction proved unsuccessful with the high school students. It is concluded on the basis of this study that command of standard English is related to school performance. (Author/GW)