I know you've already left now but still thought I'd say my tips.
When I left year 11, everyone did this, they just brought in a notebook and everyone signed everyone's with a little message. I used a arty one that I decorated and stuck in some pictures, it's a really good memory book.
Don't be too worried about it though, if it's not as big in your school ask your friends to do it with you and start early so it will catch on.
If your missing pictures from some years try to find other things to replace it like bracelets or for example if you have a good memory in a cooking class, print out recipes or draw cooking equipment.
Also if you want to make it super cute I suggest making your own notebook with different coloured paper and binding it all together :).