Your cost analysis doesn't take into account the time value of money. Being able to spread all those upfront purchases out over a monthly cost that includes IT support and the latest hardware at all times is definitely a valuable proposition. You are assuming a 4 year life on your netbooks. If one of them breaks, tough luck.
However, like you said, the usefulness of Chrome OS for students is definitely debatable. In a few years, a web-app only classroom might be more viable, but right now, it doesn't seem too practical. Plus, a tablet form factor seems more useful than laptops for classroom use.