Uniforms don't save teachers time, they create needless headaches and bureaucracy. A large part of a teacher's day is spent policing school uniform and dealing with infractions. Something as mundane as not having a long enough tie or your top button being undone can takes precious lesson time to discipline.
It doesn't save time for students either. Hundreds of students are sent home from school each year for uniform infractions. These students would be far better served by being in the classroom.
If schools really wanted to save time, they would abolish dress codes and let students wear what they wanted.