I'd say it is. My grades fluctuate constantly, meaning I go from 3s to 8s or 4s to 7s. It is really possible; but only if you work hard. For example, history is a content-heavy subject, however, you mainly get points for how you structure things (as well as the knowledge). So I'd say practice every exam question you can get or at least make essay plans by bullet-pointing what you will say. Gcses are way easier thank mocks as the it is spread over a month, as well as a may half term in the middle!!!