Following are some methods that teachers have used which you might want to consider as you create your own tardy policy. Realize, however, that you must create an effective, enforceable policy or you will eventually be faced with a tardy problem in your classroom.
Tardy Cards are basically cards given to each student with a space for a specific number of 'free tardies'. For example, a student might be allowed three per semester. When the student is late, the teacher marks off one of the spots. Once the tardy card is full, then you would follow your own discipline plan or the school's tardy policy (e.g., write a referral, send to detention, etc.). On the other hand, if the student gets through a semester without any tardies, then you would create a reward. For example, you might give this student a homework pass. While this system is most effective when implemented schoolwide, it can be effective for the individual teacher if strictly enforced.
On Time Quizzes
These are unannounced quizzes that take place as soon as the bell rings. Students who are tardy would receive a zero. They should be very short, typically five questions. If you choose to use these, make sure that your administration allows this. You can choose to have the quizzes count as a single grade over the course of the semester or possibly as extra credit. However, make sure that you announce the system in the very beginning and that you start using them right away. There is a chance that a teacher could start using these to specifically punish one or a few students - not giving them unless those students are tardy. To be fair make sure that you randomly place them on your lesson plan calendar and give them on those days. You can increase the quantity if you find that tardies are becoming more of a problem over the year.
Detention for Tardy Students
This option makes logical sense - if a student is tardy then they owe you that time. You would want to give your students a certain number of chances (1-3) before instituting this. However, there are some considerations here: Some students might have no transportation other than the school bus. Further, you do have an additional commitment on your part. Finally, realize that some students who are tardy might be those who are not necessarily the best-behaved. You will be required to spend extra time with them after school.
Locking Students Out
This is not a recommended means for dealing with tardies. You must consider your liability for student safety. If something happens to a student while locked out of your class, it would still be your responsibility. Since in many areas tardies do not excuse students from work, you will have to get them their make-up work which would in the end require more of your time.
Tardiness is a problem that needs to be dealt with head on. As a teacher, do not allow students to get by with being tardy early in the year or the problem will escalate. Talk with your fellow teachers and find out what works for them. Each school has a different atmosphere and what works with one group of students might not be as effective with another. Try one of the listed methods or another method and if it is not working do not be afraid to switch. However, just remember that your tardy policy is only as effective as you are in enforcing it.