noble and teacher I don't think comprehensives like that are exceptions. But nor do I believe the ones that do fail their most able dc, or in some cases all dc except those required for massaging figures, are exceptions either.
In exactly the same way a secondary modern is the flip side of grammar, the inadequate comprehensives are the flip side of the good ones.
I know there are many individual teachers, and slt's who do meet the needs of the most able, or at least give it the best try they practically can. However if a school decides not to bother, there is absolutely no policy, or recourse to require them to do so.