It is my understanding that children at 15 are offered access to Gymnasium/Liceo based on their performance of the previous two years. So anyone who fails that will be barred from uni, or at least have a far more difficult path ahead of them.
For an expat child who is doing extremely well in his country of origin, coming into the country one or two years before that date it might be that they are excluded from higher education just because they didn't have enough time to settle in their new country.
Would children in that situation be allowed to study in a private school and do their four years of Gymnasium there? Would the schools generaly have free spots for those kids?
We might be transfering to canton Ticino (Lugano specifically) if that makes any difference.