How can I describe what year of high school a student is in that can be understood by people all around the world?

For example, in Australia we have Year 7 to 12, which is the extent of my knowledge. Is it the same all around the world?

I noticed that in Harry Potter (UK based) they had 1st to 7th year students.

Is there an ideal way I can describe what year of high school a student is in that could be understood by any of my online friends?