Hmm...well it's hard to tell somebody else what they should write in a yearbook. I wrote a bunch of random crap to all my friends...which was rendered meaningless by the end of the year because all of my friends turned on me after I graduated sweatdrop
I guess you should just write whatever you think might make you smile to look back and reflect on later. Or some weird inside joke that only your friends get, because those can be fun.