It also means you are at the mercy of each teacher every year to differentiate the curriculum. If you get one who doesn't/won't do it, your child is effectively held back a year (or more, depending upon how much differentiation has taken place before the bad year). Plus the child does not get credit for taking subjects or understanding topics in case of transfer or moving up to the next level of school, so s/he may have to repeat subjects multiple times because the informal nature of differentiation doesn't "count" in the formal structure of the school system.
A good teacher makes a HUGE difference--the teacher can be the only difference between a dismal year and a good one, in fact!--but I really don't think differentiation is any substitute for systemic changes that support a GT child. It's better than nothing, but as a system, I don't think it's much better than nothing.
But, ah! Would that every teacher believed in tailoring the education to meet the child where s/he is!
