I’d never seen or heard of these until today:
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Those are elastic “chair bands” sold by Amazon retailer Ozio, and they’re intended for schoolkids with ADHD, ADD, autism, and/or SPD (Sensory Processing Disorder). Essentially, the child can freely fidget with the bands with their feet during class, which the retailer says helps promote calmness and increases attentiveness.
Assuming these work as advertised, it’s a great, and economic, use of materials. At fifteen bucks for fifteen bands, it’s a low-cost add for any special needs classroom, and installation’s a breeze.