A CV is a concise, factual document that is designed to demonstrate to an admissions committee, or employer, how you meet their criteria. It should be attractively presented, interesting, and targeted to a particular audience. There is no right or wrong CV, and everyone will have a different opinion on what should or should not be included, and how it should be presented. Focus on writing a CV that your audience wants to read.
The standard length of a CV for a high-school student is one or A4 page.
European CVs and US resumés tend to be shorter and contain less information about personal interests than UK CVs. A good website for European CVs is Europass where you can complete, save, amend and download your CV online. You can access it here Europass
It's good practice to prepare a CV/resumé in the style of the country you are applying to. The Target Jobs website hosts some country specific advice in addition to some useful CV templates.
Similarly, Prospects website hosts a guide to working in different countries, a guide to studying in different countries, and a comprehensive guide to CVs.