David Osborne and Anne Osborne in US News and World Report:
Creaming can take several forms: self-selection by the most motivated families; long applications that may deter uneducated, non English-speaking and/or immigrant parents; handpicking students with higher test scores; and "counseling out" or expelling difficult students.
Traditional public schools do all these things. Many districts give parents choices, and the most motivated parents choose the best schools. Many magnet and exam schools require applications and high test scores. And some traditional schools push out or expel the most difficult students. More of them ignore such students until they drop out.
Surely a few charter schools do the same. But research has found no evidence that most charters skim off the highest performing students. A 2009 RAND Corporation study found that "the prior test scores of students transferring into charter schools were near or below local (districtwide or statewide) averages in every geographic location included in the study." A study of New York City charters found that their applicants were more likely to be black and poor than traditional public school students. It also found that their lottery systems were completely random.
Research has also shown that a low-performing student is no more likely to leave a charter than a high-performing student – or than a low-performing student is likely to leave a traditional public school.