This is really going to depend on the school, but this is how it works at DD's:
If you get a sports scholarship there is usually an expectation that you pitch up to everything you are asked to attend.
At junior/early senior level DD's school tends to field 1-3 teams (and rotate children in and out of them to give every child a chance to compete). It is drilled into the kids that "it is an honour to compete for the school".
As the children get older there is more scope to do non-team sports (gym/badminton/volunteering etc) and there is less pressure to compete.
DD swims, so we drop out of about a third of fixtures. Fixtures are arranged at least a term in advance, so I tend to email head of sport with the date clashes at the start of the term.
Kids make their own way to home matches and those at schools within a couple of miles away at the weekend. Otherwise teams get ferried about in minibuses or coaches. Away matches can often be 90 minutes' drive away so it isn't unusual for a single match, followed by "tea", to take up most of the day.
The boarding schools, with Saturday school, tend to organise their fixtures in the early afternoons.
Multi-school tournaments tend to be mid-week.
Squad training is usually for 90-120 minutes after school on particular days.