Cotton is in general a poor eco choice in terms of climate change plus It takes massive amounts of water. It has a high carbon footprint.
Bamboo is a far better choice for the environment. Very fast growing and sustainable plus the processes of turning it into a fabric are less damaging for the environment
I take your point re water use but organic cotton is hugely better than conventional cotton, using 91% less than conventional cotton and is regulated by international certificating bodies.
And I think it’s important to note that not all (any?) bamboo viscose is good.
Transforming bamboo cellulose into rayon/viscose uses chemicals such as sodium hydroxide, sulfuric acid and carbon disulfide and spinning plenty of energy so it’s not without its issues and being unregulated has workers rights issues as well as environmental ones.
Lyocell, especially Tencel Lyocell, is a good cellulose product to go for over bamboo viscose as it uses closed loop production and does not require the same harmful chemicals as bamboo viscose.
Lenzing’s Tencel is made from eucalyptus using closed loop systems and responsible energy use etc etc so is a sustainable eco product.
It’s a lot of work these days to be sure our fabrics come from sustainable sources that’s for sure.
But I love (organic) wool socks myself!
Without wanting to squash his obviously kind and generous spirit OP, I also think reducing, reusing or recycling is the way to go. It’s too much to give gifts to teachers I think. Most people have enough or plenty these days. Will some teachers get 25+ gifts?! That’s potentially a lot of reusable cups.
For aunt and uncle, a handmade recyclable tree decoration maybe? Organic food gifts. Local honey?