Wouldn't rubber bands be dangerous, due to the risk of losing a limb or something?

Only in the sense that plastic bags are dangerous. If young children get the crazy idea of putting plastic bags over their heads or using elastic bands as tourniquets then they might suffocate or lose limbs. Sane people who use elastic bands as a source of self stimulus flick them against their wrist.

Also, cat o' nine tails? Seriously? What are you going to do, pull your shirt off and start self-flagellating while you're in the middle of a test or a lecture or a business meeting or something? :P XD

I thought I was safe with that one. It's a cat o' nine tails, an instrument used for brutal torture and maiming. No, you don't use that as a substitute for a bottle of mountain dew when you need help paying attention. (Yet th self flaggelation usages still fits the category 'self-inflicted-pain based learning aids'.)