Its completely normal, and it doesn't improve the schools A level results, as their success rate is the fraction of those who start in year 12 who do well in year 13.
Its is normal and reasonable to have an entry requirement to year 13. Otherwise vast amounts of time, money and resources wasted. Students who are a bit border line are accepted onto A level courses and given a chance, some rise to the challenge, some don't. Those that don't are not taken through, why would they be?
There is an alternative, if you prefer, just don't accept anyone in year 12 who isn't 100% definitely going to do well in year 13.
Of course that would exclude everybody who blooms late, or who has learnt the hard way in GCSEs, and determined to turn over a new leaf, or anyone ill, or in difficult circumstances, but hey, if we have to all cover our backs from these so called accusations of not acting legally, than fine, lets just dump thousands of teens down the kazi without a second chance, so we will all be safe