There are now many imitators, but my alma mater originated the winter carnival, and with that the snow statue (and snow statue competitions).
Having built a few and watched many more being built, if you want a bench, use sheets of plywood as forms - 2x4's with dowels (or something similar) for spacing of the forms. Pack with snow and water it down well every 8-12". You can use a little hot water as a release agent for those forms - then you can build a back rest on top of that in a similar fashion.
Any kind of saw (gas or human powered) works for any shaping, and irons work well for smoothing (no kidding - never heard of an electrocution, either

). And you could throw straw on the seat for insulation (keep the butts warm and the ice cold).
Sounds like a good time - be sure to snap a few pictures!