Do you think it's silly to want to marry someone when you're in highscool?

Um yeah. Generally relationships in highschool are complete jokes. People will be dating for like 2 days and be "omg I'm so in love with you" and then break up the following day. I don't think people should be seriously thinking about marriage until their out of highschool. At least finish your basic education first before you go making decisions like that.
It's a little bit different when people are dating for a long period of time, like a few years (ex: started dating in grade 9-10 and are thinking about it in 11-12) because at least they've had time to develop proper and (hopefully) healthy relationship.

Ask anything you want, go for it.