Google Cardboard is a free app that runs on most smart phones and iPods running iOS 8.0 or later. There is a long list of compatible apps from race-car games to Google Street View and this is where the magic happens. In order to use the app, you'll need a viewer. These range from the simple, folded cardboard devices the program is named for, to higher-end, durable viewers made from molded plastic or metal. The idea is simple and emerged in the 1800's with the invention of the stereoscope.
I bought a simple cardboard model online and was using it within minutes of opening the package. It takes a minute to get used to virtual reality, it can make you a bit dizzy or nauseous and some may notice that it bothers their eyes after a few minutes, but the effect of standing in the middle of Venice, Rome, Paris, or even under the sea at the Great Barrier Reef is worth the odd sensations that come with VR.
The cardboard model I bought only cost $9.99 and does a perfectly acceptable job, but it is cardboard. By the time I'd passed it around the 8th grade French class, the edges were looking a little grubby, so I ended up duct-taping it to reinforce the joints and protect the surfaces. But, once you've bought the components (lenses, magnet, NFC tag) you could easily replicate the structure and reuse those parts to save money. Alternately, I'd suggest buying a few of the more durable (and disinfect-able) models and using it as a small-group station. Maybe buy a few and check them out from the school's library. The potential experience is worth the small investment and as time goes on, the apps available with certainly increase in number.