Held back after class

Alexander is in week 3 of a new round of swimming lessons. He is 3 1/2 and is a good swimmer so is in a group with kids probably 4-5yrs. Whilst his swimming is almost as good as the other's his attention span is not! so while all the other kids are having their turns instead of sitting on the mat and watching the other kids (as is the 'rule') he does somersaults and generally plays under the water. At the end of yesterday's lesson the teacher told the other kids to go and 'Alexander would you just stay for a bit'! No! I thought, he is already being held back after class for not concentrating! Is this the beginning....