Scratched leg on slightly rusty bench. didn't bleed but scratched/broke skin. do i need a tetanus shot? last vaccine was at 15yo. rubbed alcohol on it

The far larger risk: is from rusty nail which has been buried in soil, is damp, & produces a deep dirty puncture wound containing necrotic tissue which allows spores of Clostridium tetani to grow and produce the neurotoxin. Study: http://www.who.int/immunization/topics/tetanus/en/index1.html. A superficial abrasion from something dry is likely comparatively quite low risk.

Maybe.......: Having had a tetanus shot 5 years ago, you should be protected. As the nail was rusty, this is considered aa tetanus-prone wound and you should discuss the possible need for a booster with your MD.