Making an LED light blink seems to be one of the easier beginner projects for Arduino, so this would probably be a good starter if you're new to all this. Another good thing about this project is that tutorials and demonstrations are all over the internet. So if you run into trouble, you could probably find someone to help you fairly easily.
This Lynda.com course takes you from installing Arduino on a Windows or Mac computer, through making a robot and wearable tech items. I like Lynda.com because you can sign up for a free trial and cancel at any time. The instructor states that no programming knowledge is necessary for the course, but that (of course) if you have programmed before, it will certainly be helpful. Lynda.com cancelled my membership with no hassle, so I wouldn't hesitate to try the course.
This tutorial video series by Jeremy Blum starts from the very beginning and assumes no programming or electronics knowledge whatsoever. I found the first video very easy to follow and understand, because Jeremy explains every step in plain English. I liked the fact that he paused after every line of code and explained exactly what the code was doing. I think a teacher could follow this tutorial series and then lead her high school students through the same process.
This lesson plan will take at least two class periods, and probably longer, to finish. The target age is 14-18, but after reading the lesson materials, I think older middle school students could probably handle this. The objective is to make an Arduino turn lights off and on. The code is provided, so the students could copy and paste it. This looks like a well-planned and sequenced lesson.
Sylvia is a truly amazing middle-schooler with her own blog and book, both featuring all kinds of Arduino projects for both kids and adults. She's spoken at a TEDx event, presented at conferences, and even impressed President Obama with her WaterColorBot project at the White House Science Fair. She's very talented. If you're considering Maker/Arduino projects with your class, you should watch a few of Sylvia's videos.
This article is exactly what it appears to be: an explanation of what the Arduino is and how it's used. The suggested readings will be helpful to anyone who needs a bit of background knowledge about electricity and basic electronics, before diving into programming one of these. If, after reading this page, you think it could be valuable for your students, work your way through the rest of the collection for some amazing projects. Hint: It's not that hard. Really.