It's just the way it goes today.

Dear Student,

You are late. Again. I would not be so fussed by this except that you're never late for any discernible reason. It just happened. I would also not be terribly fussed by this, except that you then linger into the next student's lesson, taking your time removing your presence (let me be clear, you're welcome to stay as long as you like, but no-one should notice you) and generally hanging about. I'm beginning to understand why you were late.

What I would like to tell you: It's rude. Please stop. Schedule a later lesson time, plan to leave your home five minutes earlier, pack your stuff in the morning before you leave the house. Pack it on Sunday night, since I know you're not a morning person and you don't practice before school. If I was your parent I would ask you to pay for every minute of lesson time you DON'T receive because you found something more important to do on the way. Ooh, you're here. That'll be $10.