Can teachers make a policy that students cannot attend the first period if they are tardy?

No, unless they take the matter directly to administration which will not come to a direct decision immediately. This will most likely circulate for some time as they observe and address the problem.

Also, if students miss the first period he/she could be missing out on the school curriculum or missing some other essential event/class/announcement. Not to mention, making a student sit out during a period because they're late contradicts productivity and the whole point. Why make a student sit out and miss things when a teacher is essentially making them absent from school academics (as it would be if they were late or not at school).

In short, a policy such as that would be rather pointless and unproductive in my opinion. Hope this response help in some way! :)