Is it possible to explain a transistor (or vacuum tube or relay) using a marble-based switch for students?
I'd be thinking of something like this: 

The top hopper contains blue marbles (so you can see where things come from). A spring loaded flap prevents anything falling.

Drop a red marble into 'base' tube ( = small base current) - it opens the flap momentarily, marbles drop from the 'collector' hopper and the red ('base' marble) and blue ('collector') marbles accumulate in "emitter' container below. 

Flap then closes due to spring action (that's the tricky mechanical part).

The gain of the 'transistor' is then determined by how many blue marbles fell for a single red marble. Analogy gain = collector current/base current.

We also can see Kirchoff's current law in action - 

the number of 'emitter' marbles = number of collector marbles + the base marble

or  the analogy  emitter current = base current + collector current

Hope this gives you some ideas.