What should I aim to do in my free composition to guarantee high marks?

At GCSE level, your examiners are not looking for a particularly complex piece of music. They are simply looking for a well-executed, well written-out simple composition, and the easiest way to accomplish this is through writing a pop song, though the following tips apply to other types of composition as well. As long as you can keep variety in your chord progressions (i.e. not using the same progression throughout the entire piece), write appropriate melodies and vary texture enough throughout you should be able to easily score good marks.  To unlock the highest marks, you should aim to include a simple modulation (key-change) in your piece, even if you would not otherwise include one. Furthermore, while it is not supposed to be a factor, the quality of your recording is key, as is presentation. Examiners will favour pieces that are recorded properly and not simply bounced from Sibelius, and a well-organised score/write-up will lead to gaining the highest marks possible.