My local YMCA branch does the same thing, all minors are required to have a red, yellow or green breakaway necklace that indicate swimming level - kids with red bands cannot be in anything but the 1'6" kiddie pool without an adult within arms reach (parents still have to be supervising regardless), kids with yellow bands cannot go in the deep end of the pool but can be in the shallow end alone.

It's a really smart idea, and while even an excellent swimmer can become fatigued the more experienced ones will be less likely to need aid than the less experienced ones.