In Bridge to Terabithia we see the social class distinctions and the distinctions between the fifth graders (and even younger kids) and the seventh graders. Just as Jess and Leslie's parents are separated by how much money and education they have (the former, very little; the latter, quite a bit), students at school are divided by even firmer, unspoken class distinctions. The older students get all the resources and get to abuse the younger students. The younger students have to take care of, and amuse, themselves. But maybe if they'd been given more resources, Jess and Leslie might have been less inspired to create Terabithia.
In Bridge to Terabithia, we see that money can provide education, ability, and opportunity, but even it can't prevent tragedy or protect us from being different from others.
The five Aarons children's varying attitudes prove that nature is stronger than nurture – they all have the same background and disadvantages, but only Jess and May Belle make attempts to rise above it.