Paracord (available at any craft store or online). I recommend purchasing a small kit which includes buckles and other tools. 
Students will need a 10 ft length of paracord and a buckle or keyring.
I highly recommend a small paracord bundle for beginners like this one from amazon that usually costs $10 or so. https://www.amazon.com/WEREWOLVES-Paracord-Bracelet-Crafting-Instruction/dp/B09BKJLX2P/ref=sr_1_34?crid=3434I0BKDAZES&keywords=paracord&qid=1667027286&qu=eyJxc2MiOiI3LjE3IiwicXNhIjoiNi43NyIsInFzcCI6IjYuNTUifQ%3D%3D&sprefix=paracord%2Caps%2C163&sr=8-34
This doesn't provide military quality paracord, but it is perfect for friendship bracelet making and beginners.  If students remain interested and get more serious, higher quality paracord and survival bracelets can be considered.
You can also purchase a kit that contains a jig, which also comes with paracord and makes things much easier by keeping the bracelet tight and the correct length while you work.  An example can be found here: https://www.amazon.com/dp/B077S3C8TG/ref=sspa_dk_detail_3?psc=1&pd_rd_i=B077S3C8TG&pd_rd_w=Yu1Ub&content-id=amzn1.sym.dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_p=dd2c6db7-6626-466d-bf04-9570e69a7df0&pf_rd_r=5YVCH7P40BY68Y019711&pd_rd_wg=3S8r8&pd_rd_r=fab6f063-10e0-4acb-a563-49f8978dd934&s=home-garden&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&smid=A3QZ39CEBSLY46&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzOVFDVE5YMjNSRU5VJmVuY3J5cHRlZElkPUEwNjA1NTk1MVdVUkZGWjNRV0xZRyZlbmNyeXB0ZWRBZElkPUEwNjk5NzUzQUswNUc2RVVCTUk2JndpZGdldE5hbWU9c3BfZGV0YWlsX3RoZW1hdGljJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

Students will need parental guidance to use lighters (butane or torch style preferred) to melt and seal paracord ends.