depends how good the grammar is. in my area, the facilities were actually quite a bit worse than the comps, though there was a far better selection of academic GCSEs/A-levels. If you are able to focus in lessons, then the only problem would be possibly not learning high-grade topics in e.g. maths and sciences or not learning them well, (since they might be focusing on getting people to just pass) where you'll just have to make up the shortfall yourself. Once you're done with GCSEs, you can change to a school you prefer. Plenty of people came from those comps to do A-levels. (one decided to cut science A-levels at the last minute, or something similar)
This aside, I think you are probably overestimating how academically engaged most people are. Troublemakers usually disappear after GCSE or mellow out, but you will be surrounded by people who "just get on" and are not particularly interested for a while longer, definitely still in A-levels and perhaps into university as well. (many people go to university as a natural progression for school and just pick their best A-level subject) I think you mainly find the environment you're seeking at very high-flying selective schools, and hopefully at university if you choose to go.