They're Not Stupid

From The Power and Peril of Praising Your Kids:

Psychologist Wulf-Uwe Meyer, a pioneer in the field, conducted a series of studies where children watched other students receive praise. According to Meyer’s findings, by the age of 12, children believe that earning praise from a teacher is not a sign you did well—it’s actually a sign you lack ability and the teacher thinks you need extra encouragement. And teens, Meyer found, discounted praise to such an extent that they believed it’s a teacher’s criticism—not praise at all—that really conveys a positive belief in a student’s aptitude.