How about write something along the lines of,
"It's been such a great year! I hope that you'll be in my homeroom (again, if they already are) or at least a class next year. But let's not think about school, because summer's almost here and it's time to let loose and PARTY! You're so (fill in with a few compliment words, DON'T use 'nice,' it's too boring and insincere. Like maybe say 'original,' 'funny,' 'sweet,' 'witty,' 'fresh,' 'psycho,' 'loco,' 'kind,' 'fun,' 'fabulous,' 'stylish,' or 'wild.' Whatever fits. Only use 'psycho' if it's a weird boy that won't mind.) See you next year! Hope you look forward to having to put up with me again."
Just do something that isn't too short and people will enjoy reading. Like, because it's personable. Or if you really don't like the person, say something like this.
"I hope you have a nice, relaxing summer. Enjoy those few months of freedom while you can."
Just something very plain but still nice. I hope I helped, and apologize for the long post. Well, all my post turn out to be long.
