To offer a different view... DS went to MK at 17 in y12. We pulled out of a poor choice for Y12 and repeated 6th form. The experience was very mixed. In the end, DS did much better than he, or we, expected and completed Devizes-Westminster, most of the DoE requirement and came out with three better than expected A level results.
We did not find the school helpful, at all, when we raised questions and specific teacher requests were ignored in favour of the school's convenience, but if the student pursues a teacher their persistence is generally rewarded.
As in most schools, some teaching is great and some isn't. We had better results in Maths and Physics than we expected, but the third subject projection was shown to have been a bit over-optimistic come results day. So, overall DS was enabled to do better than forecast, and the outcome was okay.
There is currently a lot of disenchantment among local professional families who should be important customers for the school because of the emphasis on the national swimming coaching.
It's probably worth saying that there is a significant paralympic programme which does well, and that the Chinese students (who come to learn English fluently to study in the UK/US and Australia) occasionally make acquaintances with locals.
It's not an Oxbridge mill, although there are some candidates most years, but the academic students go to decent RG universities. If your child is clever and local, choose Exeter School in preference. But for your middle-ability DC, if the location suits and the approach, you could do worse. The academic side is equivalent to a good comprehensive in a middle class SE catchment (there isn't one, but Plymouth is a grammar area). In rural Devon, it may be a decent bet if you don't want boarding, although Devon is huge and MK isn't central.