Only during the Commercial and IFR navs are we allowed to lean the mixture during flight
An iteresting concept, I once flew with a guy who was taught to never lean below 5000'. Does wonders for your fuel flow in some types. I know of one type that goes from the low 30 l/hr with leaning to 50 plus if flown full rich.
I think you should be taught to lean in the air and on the ground from day one as this should breed good habits from the begining.
As for the clearing a fouled plug, if you have a CSU try cycling the CSU once or twice as the change in pressure can dislodge the carbon on the plug.