They're good, and since the town has one state secondary, it needed to be. The village primaries post solid maths results. Most of the county's schools are rated good or better. The famous school in town charges fees. There isn't much choice, but what's there is decent.
There are 11 state primary schools around here, and their average Key Stage 2 maths score is about 105. In town, the obvious names are Marlborough St Mary's CE Primary and Preshute Church of England Primary School. Out in the villages, Ramsbury Primary School and Great Bedwyn Church of England School serve communities that would otherwise face a school run on country lanes. Most of them are Church of England schools, so expect a harvest festival or two. The results don't depend on it.
The secondary side is simpler. There is one state secondary in the area, St John's Marlborough, and its pupil-weighted Attainment 8 score is about 51. With one option, families don't shop around for a secondary here; they get to know St John's. Marlborough College, the independent boarding school on the west side of town, is the other secondary everyone mentions. It is a different purchase, paid for in fees rather than house prices, and the two are not in competition for most local families.
Ofsted figures only exist at county level. Across Wiltshire, 106 of the 128 schools with a published overall grade are rated good or outstanding, which is more than four in five. Six are outstanding, 20 require improvement and two are inadequate. The requires-improvement count is the one worth checking against your shortlist before you fall for a kitchen. Some settings have not been graded yet. That tells you nothing about their quality, only that no inspection has been published.
Buying near a school does not get your child a place. Admissions follow the council's and each school's own rules, and several primaries here are church schools with their own criteria. Distance counts, but it doesn't settle things on its own. Buyers who pay extra to live next to the school gate without reading the admissions policy sometimes learn this the expensive way. Read the policy before you make an offer.
Schools are one of Marlborough's real strengths. The primaries do well and the one state secondary sets a decent standard, so the only real downside is that parents have little to choose between.