George, We at Van Nuys High also lock out students during Period 1 if they are tardy.
Teachers are cooperative.
Considering that we have 3000 students on campus, the tardy problem is minimal. I have been at schools where the tardies are much worse with fewer students. When I have been at Van nuys long enough, I will be able to describe what the school culture does to minimize the tardies. I suspect it is my principal's constant and insistent enforcement of the tardy policy that creates the climate of punctuality.