With the exception of the afternoon prayer times for our Muslim students -there is no prayer obligation that would interfere with getting to school on time - the 10 minute rule is and always has been in effect. By allowing your students to come in at any time, you are reinforcing the idea that we are a bunch of pushovers.
Please make sure to kindly invite the late students to sit outside and study on their own. Allowing them to take a seat 30 minutes after class has begun is an insult to those who arrived on time. In turn, those students may also reconsider their start times in the future, seeing as there are no consequences.