Naive?
No, I don't think so. Admittedly we don't know much about op, as she hasn't yet told us.
But let's just speculate. Go with me on this? 
Does op herself work? at all , part time? The main breadwinner? does her husband work?
Have they ever moved house? Can they again? As part of their next 5 year plan? 
Have they ever moved into this area? Presumably they have because they didn't grow up here so there's nothing stopping them from moving again. How old are our kids? Are they primary, as other posters said things change over the years and a school can get better or can actually get worse.
Loads of Mumsnetters change jobs or take a promotion or study more go to open university and change careers, completely. there's nothing stopping op whatever job she does doing something else in the next five years, studying online getting something else, getting a higher paid job, moving to another area with better schools.
there's nothing stopping her (depending on her husband and where they live) moving to a a different county, applying grammar or better schools.
or even if the school is semi reasonable She could invest in her child. She could do online learning. She could push them, encourage them, she don't even need to pay for a tutor if you have the time energy and expertise you could practically look it up and tutor your own child to make sure they got decent GCSEs.
Admittedly I'm teasing here. My dc didn't go to grammar, I didn't get tutors. but I'm just saying she doesn't even need to necessarily move to a better school.
as long as the schools around her are semi reasonable, a bright child with invested parents will thrive.
What's to stop her dc getting very good GCSE results , good A-level results and going to after university? absolutely nothing, or rather hopefully, not a lot.