Madison Teachers, Inc. Newsletter, via a kind Jeannie Bettner email (PDF):

Rights granted to an employee by the Union Contract are among the most important conditions of one’s employment. Those represented by MTI, in each of MTI’s five bargaining units, have numerous protections based on SENIORITY. Whether it is protection from involuntary transfer, being declared “surplus” or above staff requirements, or layoff, SENIORITY is the factor that limits and controls management’s action. Because of SENIORITY rights guaranteed by the Union’s Contract, the employer cannot pick the junior employee simply because he/she is paid less.
Making such judgments based on one’s SENIORITY may seem like common sense and basic human decency, but it is MTI’s Contract that assures it. Governor Walker’s Act 10 destroys these protections. MTI is working to preserve them.