A simple checklist to post on your door or outside of your classroom to remind students to enter appropriately.
I work at a school that requires uniforms and bans certain items such as hoodies and headbands. This acts as my reminder to students to remove any non-uniform items, and to enter my classroom prepared, calm, and ready to learn.
Students know that when I stand at my door and say, "You're cool." It means I recognize that they are ready and acting appropriately. Conversely, if I say, "Stop. You're not cool yet." They know that means they need to step to the side and check themselves.
It is short, simple, memorable, and is a fun play on words, particularly for older kids. Many of them even say, "I know I'm not being cool," and will self-correct.