Here is a great article by Katy Merrell, a school Head in Norfolk, Virginia, Pro-Active Parenting, The Power of Play-Based, Active Learning. She says, "too often the word ‘play’ makes parents apprehensive about what is taking place in the classroom, so it may help to think of it as Purposeful Learning (that is) Active (while kids are) Young (and young at heart)."
When children get the "Ah ha!" moment in a learning situation, it engages the neurotransmitters in the brain that imbed the memory. A mind-numbing worksheet just doesn't have the same impact. Exciting and engaging activities that pull children in, the kinds of things they view as play, are the ones that stick with them best. Piquing and supporting children's curiosity creates optimal learning.
For older children "playing" (exploring) with a microscope to see what's in pond water, or cutting and gluing as they figure out how to make a better wind turbine blade, feels like play - and that's what the teacher is aiming at. Great teachers use play as the hook to help children push themselves to reach the highest of expectations.