THE GOOD: HSA is an average prep school largely attended by a local community mainly because it is convenient and has good facilities.
THE BAD: The merger between St Andrews School and Halstead School has not been as smooth as it should have been. The overwhelming feeling is one of organisational chaos: three headteachers in three years - make up your own mind!
THE DOWNRIGHT AWFUL: Where the school completely fails is in its understanding of the needs of neurodiverse children. To date, it has on any available metric (SENS etc) consistently failed any child even remotely near on the spectrum. Unfortunately, it is still all too happy to consider neurodiversity as poor behaviour rather than a medically recognised condition - and this is largely to compensate for the school's own inadequacies.
So, if your child is even remotely near or on the spectrum, whatever the condition (ADHD, dyslexia etc), in my view this school may not for you, until it catches up with the rest of the world. In this regard, it is certainly not a progressive school in any way.
There is an abundance of post COVID-19 data which shows that many more young pupils are affected by neurodiversity and the increasing prevalence of the issue is matched only by the school's drive to bury its head in the sand.
SUMMARY: Perhaps shortlist rather than dismiss if you are not too fussy. There are plenty of other great schools in the area.