Two light boards (learning and show) NOT running at same time?

Maybe you need a DMX merger ?...
www.thedmxwiki.com/.../dmx_merge

Yes, this should probably be okay.  Before lighting systems went Network, many systems were designed with 2 or more DMX Input plates, such as one in the lighting booth and one down in the audience area (often used for technical rehearsals).  The console would be plugged into one or the other port and would work, but if two consoles were plugged in at the same time and turned on, there could be problems.

That's a long way of saying, someone could make or get a DMX Input plate with two Inputs that are wired together inside the backbox.  I would definitely recommend speaking with an ETC Dealer about how that can be done successfully.

As arnof suggests, an active DMX Merger would be the safest, most bullet-proof option for you.  

If your college has the wherewithal, a simple Y cable could be made up with a double pole double throw (DPDT) switch changing over the two inner cores of the cables should do the job.

That would work for sure, but it might be overkill. That tool would allow me to run the lights with 2 boards simultaneously, which is beyond my needs. (So I'd like to avoid that expense.) 

It sounds like a Y-cable might serve, as suggested elsewhere here.

Something like this would be the simplest solution. https://www.newlighting.com/6840/new-doug-fleenor-dmx-a-b-switch-box-single-universe

I contacted a local sound company with this idea of a box with a switch, and he says it should be no problem to put together. Thanks.

Thanks for the suggestion. I contacted a local sound company with your description, and he says he can make a box for me with a switch to serve functionally as you describe - probably a more secure solution to have a box with switch.

Perhaps someone from ETC can tell us what the output impedance of a switched off console is, but if it is high then a simple Y microphone XLR splitter would probably do, with 5 to 3 pin adapters where necessary, providing the cable runs are not excessive. What would stop this working is one desk loading the other or signal reflection from the unused branch causing erratic fixture  behaviour.

A simple Y connection is a bad idea.

E1.11 DMX requires transmitters to terminate (and optionally ground) the line, while E1.20 RDM requires RDM controllers (like ColorSource) to have an RDM controller termination network which biases the line to prevent noise when waiting for an RDM response.

Connecting two compliant consoles at once - even if one is physically turned off - will create a multiply-terminated DMX/RDM line, probable ground loops and all the attendant signal quality issues.

All technical details aside, this is in an educational setting which makes it doubly important to be strict about compliance.
It's important to teach "best practice", even if it's usually possible to "get away" with less.

A DPDT switched "crash box" as suggested above is the simplest solution, and very cheap.

- I call them crash boxes because they make a brief nasty flicker when switched "live". I don't think that matters for this OP!

There are also a variety of DMX mergers like the one Arnof linked to, and ETC's Gateways can also be used as DMX mergers.
A DMX merger solution allows a "live" handover of control between consoles, or even to share control.