How can I write an effective, original composition which will get a good mark?

The key to writing a good composition for A level music is keeping to a specific style which you set out in the accompanying explanation of the piece that you provide with it. In choosing a style or genre a good idea can be to choose one which you have a studying as part of your course, for example a classical string quartet or maybe a jazz trio, as you will have thoroughly learnt about and will have all the notes available for the stylistic features which songs in that style are required to have. When you come to writing the piece it is always a good idea to start by writing the melody or the basic melodic and harmonic shape of the song on an instrument which you are familiar with - your main instrument. Whatever the style, by beginning to write it on an instrument you are comfortable with will make it far easier for you to naturally hear which chords it should go to next and where the melody should be headed. From here you can utilise the work you have been doing in the other parts of your course to fill in the rest, while also making sure the voice leading of each part is correct, the tonality modulates a few times, and the texture varies throughout the piece.There is an exception to writing a composition in a style you have been studying, however, and this is because your composition you should always play to your own strengths. This is true of the instruments used - try to use as many instruments that you actually play in the composition - but also in the style as well. If you write songs in your spare time, which a great many of music students do, definitely try and write one in that style, even if it is not taught as part of your course. The reason for this is that the best thing you can do to improve your compositional writing is to practice it! The more you write in a specific style the more familiar you will become with it, meaning you can add certain flourishes to it and will become more confident with your writing.