I don't have a good way for students to do it. You can use LaTeX markup in the RCE advanced equation editor with either \bar or \overline if you're typesetting them yourself, but I don't generally try to teach students LaTex so that's not really a student-friendly solution.
There are also various ways to do it in CSS and Unicode as explained here: Overline - Wikipedia , but I've never personally messed with them. I suppose using combining characters in Unicode would probably work the very most places compared to other options, but I've never actually gotten around to playing with that and I don't generally try to teach my own students Unicode either.
