Uniforms save time for both students and teachers.
For teachers, a large amount of their day is spent enforcing school dress codes. Codes which dictate the minimum length of a girls skirt, or prohibit t-shirts with offensive slogans or ban tracksuits are difficult to enforce in large schools with thousands of students all dressed in their own clothes. School uniforms save time. They have everyone dressed in the exact same way, so there is no debate over whether or not an item of clothing is in breach of school rules. It simply is part of the uniform, or it is not. Also, students that break the school uniform dress code clearly stand out amongst a sea of students that are all wearing the same items of clothing. Uniforms also save time for students. There is no decision-making process involved in getting dressed for school. Students don't need to spend minutes stood in front of a mirror to decide what they are going to wear, they simply put on their uniform and head to school. This is why many successful people like Barack Obama and Steve Jobs have several versions of the same suit. It means they don't waste time standing in front of a mirror deciding what to wear. 
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.
[P1] Uniforms save time for teachers and students. [P2] Therefore, they should be mandatory.
[Rejecting P1] Uniforms don't save time for anyone. They are a waste of time.