the pilot's failure to lean the mixture in accordance with the pilot operating handbook specified procedures, which resulted in a power deficiency, a degraded climb capability, and the inability to attain/maintain an adequate airspeed that led to a stall/mush condition while departing in high density altitude conditions. The pilot's failure to abort the takeoff when an excessive takeoff ground roll became apparent was also causal. Also causal, was the certified flight instructor's failure to provide the proper leaning procedures during a preflight briefing.