I know that when I was at school (those were the days)...even while completing VCE and a bachelor degree they were more then supportive of giving me time off to represent my country.
I don't think that decisions on when to host a World Championships should be based around the junior players availability but more so when the elites teams are available. Most sports at an elite level host there junior competition at separate time's to their senior competition. However I also think that it worked quite well in NZ to have the masters and juniors at a slightly different time. That way they can still help contribute to fees and interacte with the elite players but also pool timed is freed up to shorten the elite competition.