Hiya Linda, @flourish44,
Arduino is a great intro to technology, and middle school students are perfect for beginning that exploration. The littleBits Arduino module is a good start, as the input/output ports are limited in such a way as to direct the students in a productive direction. The DFRobot Beetle is a similar product, which trades it's lower price for littleBits ease of building a useful circuit. So I would suggest going with the Beetle if you incorporate soldering, or Adafruit's Trinket/Trinket Pro, or Arduino's Micro if you use traditional breadboards, but littleBits Arduino is ready to go with any other bits you want to use. Look into the Proto bit, as it let's you add anything you want to the circuit (except a banana - for that you need a Makey-Makey bit!)
Use this forum's search field to find all the Arduino threads. I'd also suggest searching for threads by @alexpikkert and @JackANDJude for ideas. Those two members seem to have this hardware down!
Anyway, have fun with your class!
... ps, check out forum.arduino.cc if you want to visit a site that provides a richer, yet more rough-and-tumple forum that goes both broader and deeper into the Arduino world.