Our boys generally put them on their kart bodies each year. I think at one time they covered their school notebooks.
I don't see it as prideful or boasting at all. Pride in their achievement, sure. Pride as in getting puffed up and letting it go to their heads, no. There's a difference between pride in accomplishment and sinful pride. For instance, I am proud of my children, proud of their ability, proud of my alma-mater (well at least sometimes,) but it's not a pride that boosts one's self. Sure, we can make idols out of just about anything, trophies, stickers, you name it, if you give in to that. Keep in mind, that at the Bema Seat judgement, (a judgment of commendation, not condemnation,) Christians will receive "rewards," ie crowns for the things/works that we have done in Christ's Name. That's not a bad thing. We lay those crowns at Jesus' feet and give glory where glory is due. All for Him.
Display your stickers proudly...just don't worship them.