My old school used Cognitive Ability Tests. They are a pretty reliable way of predicting future performance. If they were useless, they wouldn't be used.
Nearly all schools use some kind of reasoning tests, running parallel with curriculum assessment.
At the 95%+ working-class school I used to work at, the average CAT score was around 90. There were several obviously bright pupils with 130+ scores, who may succeed despite the surrounding mediocrity and their often unhelpful upbringings.
In my experience, high reasoning scores are a very good predictor of good performance at tasks that require novel thinking or abstraction and a good predictor of speed at clerical tasks (for example, completing a 12x12 multiplication grid as quickly as one can).
I have not yet had a pupil in the top-performing quartile of a class with a bottom quartile reasoning score.
Very few parents at the working-class school were aware of private schools and the possibility of promising offspring getting scholarships.