What is a good definition of the word "rock" for teaching?
I usually tend to think that the shorter a definition is the more it introduces ambiguity and errors. So I would just think about your audience and choose an appropriate length (You  didn't mention the level you are teaching for).

Similar to the definition of life you can offer to your audience characteristics, which some rocks will conform more or less. This is just a list I thought out:


Rocks are solids at the Earth's surface


I would definitely include solid ice, which is the primary lithology of Greenland and Antarctica
How solid? Most of them are incompressible to high pressures(Exception porous sediments)
With lower temperatures more things become rocks (even our Sun).
Density varies greatly (bring a lead-ore and a floating tuff)

Rocks are consolidated


To differentiate them from sediments rocks have underwent a certain amount of compaction and cementation

Rocks form through purely anorganic, organic, or a mixture of those processes

This category probably needs the most explanation on how diverse the pathways to a rocks are

Rocks form through natural processes

Human-made rocks exist so instead of tossing that aside, talk about the interesting differences and similarities. Natural rocks fulfill the requirments of chemistry, physics, thermodynamics. Human-made rocks additionally fulfill requirements of pragmatism: a brick needs to be square in order to build a wall; holes make it light and insulate better; etc...

Anorganic rocks fall into natural categories which obey the conditions set by their chemistry and the resulting eutectic points, reduce chemical oversaturations, and minimalization of the Gibbs free energy (There is a reason why granites in all their diversity, are actually pretty similar)
Organic rocks obey the requirements by organisms of which nutrients exist in excess and can be used to synthesize a skeleton or shell.
Rocks don't fulfill the requirements of life

Talk about what makes living matter special. Talk about, that on planets devoid of life everything is a rock or sediment.

Rocks have a scale

One atom or chemical compound is not enough to make a rock. At what scale does language speak of a rock. What scale is so big that other words are introduced. Talk about how words are also just concepts defined by other words which are concepts themselves (This is what makes science fun)