Emacs is an editor that comes built-in on your Mac (just open the terminal and type "emacs"). AquaEmacs is a slightly friendlier version that has a GUI (instead of just the terminal), and a few Mac-isms that make it easier to use. If you're curious about Emacs, but don't want to try the pure command-line version, you may want to give this a shot.
jEdit is a programmers editor that is written in Java. It is quite complete, and features integration with other tools used to build software. Might be a little bit of overkill for beginners, however.
TextWrangler is the "lite" version of the powerful BBEdit application for Mac OS X. It costs nothing, and has all the basic features (such as syntax highlighting) that students need to edit program files.