Elementary School Valentine Etiquette
Generally, in the younger grades, kids are encouraged to give valentines to everyone in the class. That way, no one feels left out. It's a terrible feeling to be sitting in class with an empty bag, while others are overflowing.
On the other hand, kids should be allowed to like some peers more than others. If your child wants to vary his valentines, I think that is perfectly fine. For instance, give cards with his favorite superhero on them to his favorite friends; other classmates can be given cards with other kinds of heroes. Teach your children how to express their fond feelings for some, without being mean or hurtful to others.