Well assuming that they aren't just looking at the covid years because things were skewed there (one school near here got 100% 9s in 2020, when they normally had a more normal distribution) then there are ways that schools could make that more likely.
Selective entry. Naturally if you only take the top 5% then you should get better results.
Sit fewer exams. I did 10 GCSEs. That was considered unusual, most people in my area did 8. Dd1 did 15 GCSEs or GCSE equivalent, plus Additional maths. My other two haven't done quite that many, but still 13/14.
Choose exams that tend to be easier to get top grade in. eg I believe a fair proportion of top grades in some languages are gained by pupils for whom it's their first language, making it harder to get a top grade for pupils for whom it isn't.
Those are all fair enough, but you could also have the situation where a school discourages a pupil from sitting an exam if they might get a less good result. I'm by no means suggesting this school does it, but the school I attended in the 6th form definitely did things like that fair practice to keep their percentages up.
Going back to the covid cohort:
In 2021, 3,606 achieved straight 9s. In 2020, 2,645 achieved straight 9s. In 2019, this was just 837. There was surprisingly little drop last year in 2022 with 2,193 students managing it. Will be interesting to see what numbers get it this year.
So 30 students from one school in 2019 is absolutely amazing and no wonder they're proud of that. That's 3.5% of the straight 9s.