Rosenthal originally claimed the treated group's IQ gain exceeded the control group's by 24.8 points and that the gain was persistent and widespread, but the instrument used to assess the children's IQ was seriously flawed, and studies since the 1980s have repeatedly failed to replicate a strong effect, instead finding only a weak effect in a small minority of students (5 to 10 percent), one that resets after a period of weeks.
What would resolve this A large, pre-registered classroom replication using a validated IQ instrument and blinded teacher-expectation manipulation, tracking effect persistence over a full school year rather than a period of weeks.
OpenEducational psychology and psychometricsWikipedia: Pygmalion Effect