David Kohn, writing for the New York Times:
Other research has found that early didactic instruction might actually worsen academic performance. Rebecca A. Marcon, a psychology professor at the University of North Florida, studied 343 children who had attended a preschool class that was “academically oriented,” one that encouraged “child initiated” learning, or one in between. She looked at the students’ performance several years later, in third and fourth grade, and found that by the end of the fourth grade those who had received more didactic instruction earned significantly lower grades than those who had been allowed more opportunities to learn through play. Children’s progress “may have been slowed by overly academic preschool experiences that introduced formalized learning experiences too early for most children’s developmental status,” Dr. Marcon wrote.
Nevertheless, many educators want to curtail play during school. “Play is often perceived as immature behavior that doesn’t achieve anything,” says David Whitebread, a psychologist at Cambridge University who has studied the topic for decades. “But it’s essential to their development. They need to learn to persevere, to control attention, to control emotions. Kids learn these things through playing.”