Sorry, I was replying to proudmama - that IME in contrast to her experience, the children who have music lessons (and those who miss a small proportion of other 'routine' curriculum lessons for opportunities such as sport, music, dance, chess, DT, drama etc etc - I suppose what might be termed 'enrichmebnt' activities, though that is abn old-fashioned word) are not underperforming, rather the reverse.
Oh, and we wipe the floor with pretty much all schools in most sports, but that definitely wasn't my point - my point was about the educational advantage to the pupils ... pretty much all of them over the school year ... who may miss a session or two of 'normal lessons' to represent the school in some way or to be given an opportunity to show / develop their particular talents to a higher level.
Yes, this can be done outside school time as well: I have 2 children with very rich and varied outside school lives. But by including it in the school day, we enable as many children as possible, even those whose parents do not have the time, opportunity or finances to take them to activities outside school, to develop the skills and interests needed for a broad and balanced future life (IME, having experienced both, in-school music lessions are WAY cheaoer, especially because of the ability to learn in a small group which is rarely offered by good outside school teachers)