I self-taught my Spanish GCSE, most of it on Duolingo. My advice is to supplement it by actually reading up on grammar rules and things like conjugation tables. Obviously, if you're also having lessons at school, this should be taught in class. Duolingo has definitely changed since I did my GCSEs back in 2018 and I think it's definitely a much longer course so I don't know how much use it will be in the time frame you have unless you're very committed; however, it definitely helped me a lot and my streak is still going even from back then.