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But it is still shockingly refreshing to me to see any adult be concerned that a child is not being challenged. I've not seen this anywhere (with the exception of tutors, but that's a one-on-one situation, so you don't have a group to hold anyone back).

Chess. My kids happen to attend a school that is pretty competitive on the national chess scene, so it might not be AS true elsewhere. But #1, you play at your ability level regardless of age (so, in K DS was regularly being matched with 4th and 5th graders by his coach), #2, there is virtually always room for growth, and #3, chess coaches, in my experience, are specifically looking to get your child to get to the next level and to see active growth. There is not much interest in treating everyone the same. Frankly, I find the atmosphere a little too intense, but if it's what you're looking for, chess is a good place to get it.