Measure Performance?  Begin with the avg percentile of the scores in the classroom.  Either obtain the scores from the end of the previous school year or an exam at the beginning of the year.  Then if the classroom's avg percentile increases at the end of the year the teacher has done a good job.  Anyway just one example.  And don't give me any malarkey about teachers will only teach to the exams -- Doctors, plumbers, electricians etc all have to pass exams including teachers.  
And FYI my BOE has 55 criteria for judging teacher performance --  everything from appearance, professional growth, organizational skills, knowledge of subject etc.  Not ONE criteria regarding whether or not the students actually comprehended or learned the required information/subject.  Which should be the number one criteria.  Not that stopping bullying, or improving a student's self confidence are not important.