cresset,
I think you have to look at why it is failing, and if there is no other choice of school then you have to get involved much more than you might prefer to.
Some schools 'appear to be failing' because their 'top line' results are poor, based on their intake. However, that does not mean that the school itself is bad, or that an able child may not do well there. For example, I know of a school where a very high proportion arrived at the school as 'low achievers' - level 3 or below at the end of primary. Although the progress those children made at the school was higher than at almost any other school in the county, this still meant they had low 'headline' GCSE results and were labelled by ofsted as 'failing' on that basis. However, their very tiny number of high achieving pupils all did very well indeed - 8 or more B-A*.
On the other hand, some schools are 'failing' because they have been coasting - they push middle or high ability pupils less hard than they might. In that case, you have to get really involved and do the pushing yourself - get the textbooks, organise the revision, tutor yourself or get someone else to do it, research which departments get good GCSE results and base GCSE choices on that, know the syllabuses etc etc.
You will also have to be really on top of any discipline / behavioural issues, and be prepared to go into school and make a nuisance of yourself.
Many years ago, DBro1 and I got almost identical O levels - he from the 3rd comprehensive intake of a tiny rural 2 form entry secondary school previously a secondary modern, me from a highly selective private school on 100% scholarship. I got more, but his levels were the same, IYSWIM. Mine were an easier ride - all taken in school, no involvement from my parents. He took 1 at a local college in parallel with school, one with a private tutor because the teacher in school hadn't taught the right syllabus, and my parents were very involved in all (DF became a governor, lots of involvement with homework etc).
It's possible, but it's harder work, getting good results from a less good school. If travelling further, or moving, is an option, either of those might be easier.