Heavy bag training can be used many different ways.
Mr. Wang, the way you describe is the most essential ways of working the heavy bag, that both beginners and advanced can use. Techniques always come in combinations. Working the bag in this way allows one to go "all out" with power...for many people there comes a point when a full-power kick is too much for a partner holding the pads.
If I were coaching, I would be careful not to limit the athlete to repetition of the same combinations...if they aren't complete beginners they shouldn't be repeating the same combinations over and over.
Here are my suggestions for (very) basic kicking drills on the heavy bag:
front leg front kick - back leg roundhouse
front leg roundhouse - back leg roundhouse
back leg roundhouse - same leg side kick
front leg side kick - back leg back kick
These are good for the most basic of training....real training should always incorporate hands.
"I'm a highly ranked officer of his tong. HE is the Dragon Head. our BOSS. our LEADER. the Mountain Lord." - hskwarrior