Karmabeliever- it's possible that a theme park visit could be curriculum based. There is a small section of the AQA Physics in Context A-Level which covers topics such as SHM, circular motion, conservation of mechanical energy, angular momentum, moment of inertia etc in the context of theme park rides. I accept that few students are likely to be studying this course though!
In response to the original question, I think threatening billboards are OTT. I'm a teacher, and I don't think that term-time holidays are the biggest cause of under-achievement, or even the biggest cause of poor attendance. Some primary schools send children home if they have the slightest sniffle, some schools spent days on activities with questionable educational value, such as nativity plays. And, as others have said , education isn't just what's formally taught in lessons and examined- even a beach holiday provides an opportunity to learn about geographical and geological features, the nature of waves, weather patterns, the tides, how to swim and observe safety rules.
With regard to the bigger picture of under-achievement- people who take their families out of school for a holiday are an easy target. Other factors which can result in under-achievement include inadequate provision for children with SEN, inadequate support for looked-after children, inadequate support for children with deeper attendance issues due to , for instance, the child being a carer, or children suffering from bullying problems. It's easier to blame parents than to accept that, in many schools, there are serious problems with behaviour management which may lead to the disruption of entire lessons. Schools are likely to be cutting teaching and support posts, increasing class sizes, reducing resources- these factors are much less likely to be trumpeted on billboards.
When budgets for schools, care support, and family services are being slashed, parents taking children out of school for a 2-week holiday are a cheap and convenient bogeyman.