Teach them, this Valentine's Day, that it's best (and good manners) to be all-inclusive, whether they like it or not. Translation: yes, your kid has to give a valentine to every person in his/her class -- not just the ones he/she likes.
Being inclusive, especially during such a public display of friendship as the Valentine Exchange, is the polite thing to do. And it's a good life lesson.
The sooner kids learn that it's best to play nice with everyone at the office, the better. And really, what's the harm in passing out cheap valentines to a bunch of people they'll never see after eighth-grade graduation?
Yes, grown-ups, we all exclude people once we're older. But being the one who's excluded is especially painful when you're a kid.
Nobody likes to be overlooked, or an afterthought.
(Well, unless that person is named "Charlie Brown.")