If you already have a garden at your school then you might like to start at level 3 of the Benchmarking Scheme. To move onto level 3 online you will have to tick yes to all of the questions for level 1 and 2 on the website.
Alternatively you can send in your evidence for levels 1, 2 and 3 at the same time as long as you can find evidence to support these earlier levels.