Would they be in a worse position leaving school before taking the GCSEs that they are going to fail?
yes, they are called resits, because they are RESITTING them, mostly, which is entirely different from SITTING them, the course is done much faster, there is much less time to do it in, it only works if you have come fresh from year 11, at least for most of the exams.
In my school for example, you are allowed onto the resit course never having taken geography before ( it is one of the GCSEs in the 5) , but you must have taken either French or Spanish, and you must have a D in it.
Colleges and sixth forms offering the EBacc typically have geography, one language, one science maths and English. Choice is limited, you may have a choice of which science or which language, or you may not.
You also need to have taken GCSEs the previous year, and have achieved the minimum entrance requirement, which is typically 4Ds or similar