Now if you really do mean that the switch itself shocks you...well, that's another story beyond my realm of knowledge.
Considering the switch is plastic...
Plastic
can conduct electricity, it's a decent insulator but with enough voltage it will. Air can be a great insulator also, think about that the next time you see lightning.
Always be careful; don't assume non metal components are safe when high voltage is concerned,
nothing is "electricity proof", just like anything will burn if you get it hot enough.
On the other side of that;
water is a great insulator, an extremely poor conductor of electricity. Don't drop your amp into your bathtub with you though, it has to be absolutely
pure H2O,
no contaminants at all, particularly salts and metals or it will light you up like a Christmas Tree.