How can I find out about the college admissions from a partciular city high schoo, broken down by ethnic group? E.g. which colleges have accepted Caucasian students from Northside Prep high school in the past two years?

The college guidance conselor may be able to give you the data, although it may not include the ethnic data you want. Many private schools publish such information. For a couple of examples, see:

St. Paul's School ~ Matriculation (http://www.sps.edu/podium/default.aspx?t=6543)
The Hotchkiss School - Academics - College Advising - Matriculation (http://www.hotchkiss.org/Academics/Coll_Matriculation.asp)

Yes, the high school counselor may have some of those figues, and/or the admission dept at that college. Of course race is not a enrollment factor ( although still tracked) it is grades, other activities, and often income levels and ability to pay.