"Padawan" is a Star Wars thing, and I'm pretty sure its origin is the prequel movies. I'm sure Star Wars fans will send numerous R2 units with holographic messages explaining otherwise if I'm wrong.
I'm not sure exactly what it means to be a padawan, but I assume it's basically the Jedi equivalent of a Hollywood intern. You know, they go get coffee, run errands, provide alibis, and are told they're lucky for the experience in lieu of a proper paycheck. Granted, I'm not sure if even full-fledged Jedi get paid. One would think they'd have to get something short of absorbing nutrients from the force.
"Grasshopper, meanwhile, I THINK is in reference to The Karate Kid? If I ever watched that movie, it was at an age when I wouldn't have paid proper attention to it, so I could be completely wrong about that.
In any case, George knows what he's referencing, and he's basically refefring to Sarah as his student in geeky ways.
EDIT - It's possible I misattributed a quote from a 1970s TV show I never watched or knew about to a 1980s movie I never saw.