@Pitagoras
"have successfully completed secondary education at an advanced level (university entrance level);"
I'm pretty sure that in all the EU countries the above statement equals highschool.
The answer to your question would be that you have to send your highschool graduation diploma (+ baccalaureate diploma or the equivalent graduation exam, and of course other diplomas that show you graduated highschool).
But if you really want to, you can send other diplomas as well, I doubt anyone will mind.