Probably did confess and rejected. Relationships like that are tricky. You're good friends with someone but something like confessing to your current relationship can break them.
Sad. Though, they're 5th graders, so it's just as likely they just got into an argument about which Power Ranger had the biggest dick and left on unfriendly terms.
@Tamerlane nah at that age I was definitely having crushes. Which was bad because I had a girlfriend. We'd kiss and hold hands and more than once she showed me her knickers. Ah, youth.