I would recommend a 3/4 size guitar. And a cheap one! You're probably looking at something that costs less than $150 per student and there are quite a few options on Amazon. I don't think we have to be really scientific with your choice of guitar. As long as it makes sounds and is relatively easy to play.

Here's the Amazon link to 3/4 size acoustic / classical guitars: https://www.amazon.com/s?k=3%2F4+guitar&crid=3BK4QMFNRYLZZ&sprefix=3%2F4+guitar%2Caps%2C138&ref=nb_sb_noss_1

This particular nylon string Yamaha (which is also available in 1/2 size) is getting good reviews: https://www.amazon.com/Yamaha-Student-CGS103AII-Classical-Natural/dp/B0050I1L9G?th=1