Fuuny how the same words conjure up different scenarios. My initial understanding of what OP wanted was a good microphone to use in class, i.e, for speaking to students, not recording. 🙂
Make sure that you have the Prof's permission to record him, or that such permission is implicit by school policy.
Other considerations after your explanation are, distance and ambient noise. You will probably want a very directional mike that will pick up the targeted speaker and minimize the collateral noise.