Barking at stranger's while on a walk

Taco Boy

Barked: Tue Nov 16, '10 4:33pm PST 
Any hints on how to sto[p my 3 yrs old from barking in obidence class or when st4ranger aproach him? He is very smart in class but doesn't like the teacher or other stranger try to approach him, he is not aggressive just barks at them and is shy