For about two years, I had students fill out their name, date, time, and reason for leaving the room or coming in late on a piece of paper on a clipboard by the door. Last year, I switched over to a online google form after Joan Le shared the idea with me at the Exploratorium Alumni Teacher Institute program. I posted a QR code with a link to the form on my classroom door so students could use their phone to fill it out on their way in/out. I also have a few computers that they can use if they don't want to fill it out on their cell phone.
I had heard someone mention in passing, at a conference or workshop, that a script could be used to email parents when a student is tardy. I thought that that was a brilliant idea! It was a way to immediately communicate with parents and hopefully change the students behavior. I had been thinking about how to do this for months, and finally created the workflow after a full day scripting workshop led by Jay Atwood at the Google Apps for Education Summit in California last weekend. Jay's guidance was key in helping me figure this out.
Linking parent emails to a form submission and sending the email is a multistep process, but I have outlined all the steps to recreate this for your classroom in this video. I hope it helps others.
Also, I want to thank Kevin Brookhouser again for teaching me how to add annotations in YouTube to make the video interactive. It is such a cool trick!