I would strongly recommend making a headstart on your AS/A-levels. The level of work required to get a high grade at A-level can be quite daunting, and it helped me tremendously to get my hands on the A-level textbooks well ahead of time, working through the content at my own pace. This meant that when I finally started term, the lessons felt like revision, and homework took very little effort, so I was better able to allocate my time towards my weaknesses, which altogether led to a much more pleasant experience during sixth form, with fewer time pressures than I otherwise would have had.