On Japanese guys' school uniforms, the second button from the top is often given away to a girl he is in love with. Its considered a way of confessing since the second button is the one closest to the heart.
Did you know that in Japan there are 3 ways to say "I love you"? You say "Daisuki" for friends and guys you like, you say "Aishiteru" for a more serious relationship and you say "Koishiteru" to the person you want to spend the rest of your life with. And they follow this rule. That is one of the things I admire about them. They didn't make "I love you" lose its meaning like us.