I was in Fiji for a little over a month teaching grade 1 at a local school. In a (hyphenated) word, the experience was life-changing! The children there are so excited to have you around and are extremely eager to learn. I think about them every day and miss them as well as all of the wonderful friends I made during my time with GVI. One of the more difficult things for me to deal with was the fact that I was so used to westernized schooling and, even though I knew it was going to be different, I was shocked when I learned that sometimes the teachers don't show up to classes. It was intimidating at first, but as long as you have lesson plans made, the students are willing to do just about anything and are absolutely WONDERFUL to work with. Granted, this happened only once in a great while for my class, but I do recommend that anyone considering this program keep in mind that they may be asked to run a class on their own. The staff and other volunteers are also really helpful if you're ever having a hard time figuring out lesson plans, using education resources, etc. One of the times my teacher wasn't in class, a few of us combined our classes for a group music lesson and it was a hit with the kids! All of the volunteers work together to make sure that the school day runs smoothly, so you're never alone...which is really nice.
Also be prepared for very basic living conditions. Cleanliness is hard to come by there, which was what I had the hardest time with.