Swimming can be very relaxing and enjoyable and one has to concentrate on the surroundings and lessons, etc. which are different; it would be too hard to concentrate on yourself and what others are doing/thinking. If you were just going to the community pool, that would be slightly different because you would be choosing to do that but with school, you have to do what they say and it is a structured class (so no one else is going to be able to pay attention to you anymore than you are going to be paying attention to them) and no one is free to joke/make fun of others, etc.