As a lot of people on here would say, take the Ofsted reports with a pinch of salt. What they tell you in the main is how good they are at jumping through Ofsted's particular hoops, not what the school is actually like.
If I had my time again, I would choose Berkeley, which is just outside Frome but takes most of its children from the town. But you have to not mind a short drive each day.
Of the schools in town, the ones which are generally well thought of are Vallis, Trinity, St Johns. I will try a brief character sketch of each:
Trinity: good at passing Ofsted (see above), but not very flexible for SEN or any other reason. Popular with 'alpha mummies'.
St Johns: slightly arty and vague and not very academically pushy. Does no harm, mostly.
Vallis: much less middle class than the other two but good vibrant school. I would send an extrovert, confident child there, perhaps not a quieter one.
All these schools have 2 form (2 x 30 entry) apart from Berkeley which is much smaller. There are also lots of smaller schools in the surrounding villages, of which Beckington, Nunney, Mells, Leigh on Mendip are all supposed to be good, but property prices are higher in the villages.
There is also a Steiner Free school, but I wouldn't touch it.