Can working hours be chagned to force teachers to bus duty?
The answer will depend on whether you have an employment contract or collective bargaining agreement through a union. Otherwise, an employer is free to change the schedule, as long as it's not done for a discriminatory purpose on the basis of sex, religion, age, handicap, nationality, or race. The Fair Labor Standards Act (FLSA) has no provisions regarding the scheduling of employees, with the exception of certain child labor provisions. Therefore, an employer may change an employee's work hours without giving prior notice or obtaining the employee's consent (unless otherwise subject to a prior agreement between the employer and employee or the employee's representative).
I suggest you contact your representative at the local teacher's union.