Will smoking weed actually make me dumber for my GCSE’s
Weed's not had much research done on it because of its legality. I remember reading somewhere about it possibly affecting the development of your brain if you're under 25. Similarly, I've read somewhere else that it can make you more likely to develop a mental illness.
In someone who isn't me's experience, weed makes them kind of hazy, depressed and generally useless. Don't smoke if you have important GCSE revision to be doing in the next couple days.
As with all drugs, nobody can really stop you from doing what you want, but at least make sure you read up on it a good amount beforehand.