The same question applies to other musical activities. For instance, Music for Youth runs its National Festival at Symphony Hall, Birmingham during term time from Tuesday 8th to Saturday 12th July. Groups from all over the country attend (having been invited after an extensive series of regional festivals), with groups from schools (primary & secondary), music services & private groups covering everything from Early Music (no, I don't mean the Beatles) to pop & urban, orchestras to trad jazz bands. A few groups are then selected to play in the Schools Proms at the Royal Albert Hall.
I've not heard yet of a school refusing permission for a child to be out of class to attend these events. Certainly, my daughters school is delighted that the children are off representing the school & county, improving performance skills, encountering other styles of music, networking and having a great time as well. It's putting to practical use all those hours spent in lessons & practicing.
In Gloucestershire, although the County music service have in the past to organise overseas tours for the Wind & Youth orchestras during holidays, more recently they have only been able to book them during the last week of the summer term. Although last year it meant 4 girls missing the school junior concert, school seemed OK with the girls going away. It was, so I'm told, hard work (3 performances in 3 separate venues on one day alone), and my daughter came back far more confident in her playing than when she departed.
I suspect that representing school / county / nation in sports events would be the same.
All that said, we've never taken our girls out of school for a holiday - except for one half day (well, leaving 1 hour early on the last day of summer term when they were in Y's 1 & 3 respectively). Not that we can often afford holidays during school holidays