Tweedpipe

Nobody suggested that the brevet was mandatory, but as correctly stated it tends to be "the key to going up to Lycée".
If somebody 'had' to go abroad, say sent by their employer and had no option (happened with me on 3 separate occasions), and if they had no other choice but to take a 14 year old with them, your suggestion of alternative schools may be worth considering.
I still maintain that if the choice is open, NOT to take a 14 year old and throw him in at the deep-end. Each child will react in a different way, but even if one does manage to 'swim against the tide', the potential future drawbacks - often emotionally or psychologically, could be dramatic.
Following one move, our son (at 7 years old) who appeared to put on a very brave face at leaving his best mates behind, began to experience psoriasis a month later - a condition that often remains with the sufferer for many years afterwards, together with considerable anxiety. The son of a work colleague in similar circumstances reacted in a far different way, which in the period of one year led to dysfunctional behaviour which ended tragically. Do NOT underestimate the difficulty that children (and animals) can go through with a major change to a contented routine.

As this is an expat forum, it could be interesting to have a specific subject on the general forum, on how children have coped in a new environment, together with a change of language at a 'delicate age'.

dmu

As TP says, it isn't mandatory, but a Lycée is hardly likely to accept a pupil into 2nde who doesn't otherwise present a solid dossier built up during the 4 years of Collège.
I've no idea of the fees for Montessori et al, but the OP is keen to avoid paying private Lycée fees. Apart from that, are there many such schools in the area in which they are interested?