Essentially, not allowing a pupil in a school sixth form to progress from one year to another during a phase of education (in this case, where the pupils have been admitted into year 12, for Key Stage 5, with the end point being the sitting of their A level exams) amounts to a permanent exclusion.
Permanent exclusions are only allowed as a behaviour management tool. They cannot be used to 'punish' poor academic achievement. (and no, not allowing year 11 pupils to join the sixth form, the school's post-compulsory school age offer, at the same school on the grounds that they have failed to meet the published admission criteria isn't the same thing).
The relevant Government guidance doesn't apply to all settings, though
This guide does not apply to independent schools (other than the academies listed above), city technology colleges, city colleges for the technology of the arts, further education colleges, sixth form colleges,16-19 academies or 16-19 free schools, all of which have separate suspension and permanent exclusion procedures.
Suspensionsandpermanentexclusionsguidance.pdf
Yes, it used to happen a lot, but the publicity around the St.Olave's case (legal challenge by the parents of some of the pupils affected) was a real wake up call for schools. Around us, there was a quiet but swift rewording of sixth form policies. DD was moving on to sixth form that year; a friend of hers in the year above had just been told that she would not be allowed back to enter year 13 (and to be fair, this was at the local grammar school most likely not to enforce the AS level requirement rule, so I don't know how bad her results were) and had actually secured an alternative place, when St Olave's hit the news and I suggested that DD should prompt her friend to mention it to her former school. Her place was reinstated on the spot...