Term in Belgian school

All the decent schools are full, and given the differences in curriculum, it would be difficult (to say the least) for him not to loose a whole term's worth of education.
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Besides - the school would not get funded for this, so I'd expect the answers to be negative, except for fee paying schools.
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What might be a better idea is a summer swap with someone with a similar aged teenager. A few weeks in a French speaking family where they have a bit more time to devote to the French aspect should go a lot further than a busy school term.