This is a good portable speaker for iPod. It's called the Logitech mm50 (in Japan: Logicool mm50). It costs 15,800 yen (www.apple.co.jp, click Store, use search.)
If your classroom is a language lab with AV input in the teacher's console, you can just use an AV cable (previous slide) to connect your iPod into the LL system.
Likewise, if there's a TV in your classroom, you can just use an AV cable to connect to the TV, and rely on the TV's speakers for sound.