I'm with mAlice on this one. Asking to pay restitution is fair. What isn't fair is that one child should get off because their parents could pay the fine, while the other was not. If it couldn't be paid then the student should have a way to work it off. One student has parents who can buy their kids way out of punishment, and that isn't fair. In fact if it was me, I can BET my mom would have refused to pay it and made me either find it somewhere or work it out on my own. And at 12, making 100 bucks isn't exactly easy.
Don't mix fine with restitution though. They were asked to pay restituation which was intended to make the "victim" whole (IE - replace her shoes). One party agreed to do so - the other didn't (because she couldn't afford it). This isn't a fine. They weren't being penalized for an infraction (in other words, they aren't following some fee schedule as you would with a speeding ticket or trespassing violation). They were asked to pay their share of the replacement. And just because the one girl couldn't, shouldn't mean she should be let off scott free.
I remember, as a kid, throwing rocks over a moving train. We were trying to land the rocks on the passing carts. Fun game for a 10 year old. However, once the train went by, we realized that there were stopped cars on the other side of the train that weren't there when the train first came through. As a 10 year, I didn't think about the possibility but certain didn't intend to cause damage. The windshield on the one car was cracked so the driver pulled over, asked for our parents and pretty much said they expected to be paid to replace the windshield or they would involve the police. My parents were asked to pay restitution in leiu of handling it legally. That is far. If my parents said no (either on principle or because they couldn't afford to) - I would've suffered another punshiment (whatever the cops deemed valid). Thankfully, my parents covered the deductible which I, in turn, had to work off in allowance/chores.
More to the point, the ability of a party to pay should be a non-factor. Otherwise, everytime a similar scenerio came into play, I would simply say "I am broke. Sorry."