The best advice here is for me to walk you through step by step how to write a letter of appeal. Remember firstly that you need to consider what your grounds are on this appeal. For example: Are you claiming against an unfair process, or medical issues? It's important that you gather any evidence you might have. If you have a medical note, attach that with your appeals letter. Make sure however that any information you give is accurate and relevant to the appeal. If you feel you have the grounds for an appeal, then it's time to start your letter. You need to adopt a formal position without too much emotion, so don't be overly frustrated or angry in your letter. Start off by dating your letter, and writing your address at the top so that the appeals body can contact you. If you know who you are writing to, you can start your letter like "Dear X,", otherwise use "Dear Sir or Madam,". Firstly you need to outline the purpose of this letter, for example "I am writing to appeal for the school place of my child". Now you need to detail all your evidence in your favour: "I believe that my child has been treated unfairly because of his/her medical condition". You can then go on to provide relevant medical documents, and any other information you may have. Finally, you should end the letter by writing "Sincerely," followed by a signature and your name. In the case where you are appealing because your child was down by a few marks in an entrance test, I recommend that you provide evidence of other tests and exams your child has sat. Notably those where he or she scored highly, and then go on to detail any specific reasons why they did not do so well in the admissions test (for example family issues etc). Here are two examples of appeal and complaint letters which will show you the style you should adopt:
Lastly, good luck with your appeal!