What's the message?

So lately I've been thinking a lot about writing.  Truthfully I spend a lot of time thinking about writing, when I'm not thinking about my family and my job, the rest of my energy is tied up in writing. 

Because I'm a teacher I want my students to learn something when they read.

But when I look back at my own novel, there's not a clear lesson.  I never focused on a theme, or a motif or any of that stuff.  I focused on telling an entertaining story, realistic characters and the occasional joke.

My book is about a boy who is a terrible boyfriend and doesn't see that he's a bad boyfriend.  He's not abusive or mean to his girlfriend, just absent most of the time.

He also punishes the town bullies, in less the legal means.  But somehow it's justified because he's doing it to bad people.  It's a vigilante story... but he still does bad things and goes unpunished for them.

But I really don't want to change the plot either.

I should also point out that my students are not my audience.  I am gearing my book for an older YA crowd. 

So what do you think?  Does every novel need a message?  Or can an entertaining story be enough?