It doesn't help that there are little healthy options in local schools. Most schools have a single privatised canteen that offers a lunch option (typically chicken curry and rice or chicken chow mein) with the rest being soda drinks, chocolate bars or salty chips. I don't remember apples or bananas being an option unless you brought it from home. But you don't want to be the kid eating a banana when everyone around you is guzzling RC Cola (the supposedly healthier alternative to Coca Cola).
Pacific Islanders like to eat. I think it's ingrained in their culture. Before you call me out and ask for a citation, let me just say this is based on my experience as a Fiji Islander. But maybe there's no problem with eating a lot, if what you eat is healthy. So instead of reaching for the local produce, the cheaper and more attractive "Western" product wins out. Plus, it's just a tad more cheaper.
P.S. It's very easy to buy Singles or Loosies (sp?) in Fiji.