As of today, lots of people with kids will head to Brussels airport (or pack up their cars) to go on holiday abroad.
The reason? Well, it's a lot cheaper to travel before the big summer vacation starts. Tour operators charge less the days before a school holiday and most parents don't think it's important for their kids to attend school to the last day.
For high school students, the exams will be finished and afterwards nothing much remains. The teachers need time to correct these exams and deliberate the results. Then there's a day in which books for the next school year can be obtained, and old books handed in. So nothing important really (not for the students, anyway) and that's why parents think they can just as well leave a couple of days ahead of the general holiday - and save money.
Alas, we poor teachers can't do the same! I really think it's not correct that prices go up when the school holidays begin. That way we always have to pay the lot, and we'll have to wait until we're pensioned (whenever that will be, as our government can't seem to form an agreement on this) to pay less for our travels.