The Wireless Microphone will give you freedom to move around, especially if you are screencasting during class. There are wirless headset microphones, but those won’t be real convenient if you are using it in class (might be more of a disruption).
Examples include:
- Logitech ClearChat Wireless Headset Microphone ($80 @ Amazon.com)
- Revolabs xTag Wireless USB Microphone ($195 @ GoElectric.com)