Better than the town's reputation would lead you to expect. Roughly four in five graded schools across the county come out good or better. Three hold the top grade. Twenty sit in the middle band. The odds here are decent, and the work is in which building your child actually ends up in.
Ofsted has published grades for 120 settings across Cornwall, the authority St Austell belongs to, and 95 of them are good or outstanding, which is roughly four in five. Three hold Outstanding. Twenty sit at Requires Improvement and five at Inadequate. A further 113 settings carry no published grade at all, because inspection has not reached them yet, and they sit in neither column rather than quietly in the bad one. These counts describe the whole county rather than this town alone, so read them as the shape of the odds, not a verdict on your nearest gate.
Living near a school is not the same as being offered a place at one. Admissions are decided by the admissions authority against published criteria, oversubscribed schools turn families down every year, and estate agents have never once been the people who allocate places. If a particular school is the reason you are buying a particular house, confirm the current criteria with the school and the council before you exchange, not after the offers go out in the spring.
The town runs two comprehensives of its own, Poltair School, which was the grammar school before comprehensive reorganisation, and Penrice Academy. Brannel School serves the same area from the village of St Stephen-in-Brannel, and all three sit within the Cornwall Education and Learning Trust. Post-sixteen, Cornwall College is based in the town at John Keay House, which is also the headquarters of the wider college group, so staying local for further education is a genuine option rather than a consolation.
Primary provision is spread thickly across the town and the clay villages, from the estates on the edge of St Austell out to Charlestown, Foxhole, Penwithick, St Dennis and Stenalees, mostly small enough that the head knows the parents by name and probably by car. The outlier is Sky Primary and Eden Project Nursery, a school that has hitched itself to the county's most famous former clay pit, which is either genuinely inspired or the most Cornish branding exercise ever attempted. Either way, it is not a school that struggles to explain what it stands for.
Schooling is one of the strongest cards this town holds, and the county-wide grades back that up without needing any help from the marketing. The thing to get right is not whether the schools are good. It is which one you can actually get into.