For one thing it may be unnecessary use of antibiotics since not all children exposed will get the strep infection. Secondly there is some concern that if strep is treated too early, a child will not develop antibodies to that strain of strep and may develop a carrier state. They have enough antibodies not to get sick themselves but they can pass it on to others. If this situation develops, your pediatrician will explain the various treatment options available. The strep bacteria remains very sensitive to most penicillin or erythromycin antibiotics.
True resistance is rare.
If treatment failures occur it may not be that the antibiotics aren't working. After a strep infection is treated the enlarged tonsils might remain for some time and are available like catcher's mitts whenever the next child in class coughs the strep bacteria nearby. Also some infections may remain deeper in the tonsils and a 10 day course of antibiotics is not enough. If your child is having repeated strep infections your pediatrician will discuss the treatment options with you. Each case is decided on an individual basis, but generally a patient must have at least six documented strep infections in one year or significant obstruction of the airway by enlarged tonsils before qualifying for a tonsillectomy.