"Learned of" or "Learned about"?

When would I use "learned of" versus "learned about" in a sentence? For example:

The principal learned of the planned protest and suspended any students wearing the armbands.

or

The principal learned about the planned protest and suspended any students wearing the armbands.

The two prepositions seem to indicate different depth or specificity of knowledge.

Learning of something is similar to hearing of something; it implies a vague or general knowledge.

Learning about carries the meaning of having specific knowledge about something. Again, compare to hear about.

Lukewrites' answer is correct; I want to expound on the exact meanings:

• To learn of something means to discover its existence (the depth of knowledge is restricted to knowing the thing exists). For example, if I learn of the meteor shower tonight, all I know is that there will be a meteor shower tonight.

• To learn about something means to discover any amount of knowledge of the thing (the exact depth of knowledge is determined by context). This could be only its existence, or it could be a cursory understanding, or it could be the result of study. So if I learn about the meteor shower tonight, I might only know that a meteor shower will happen tonight, or it could be that I have learned that it's the Orionid shower, _{^{and that the Orionids, like all meteors, are composed of debris particles which have been dispersed from Halley's Comet, and that Earth's path intersects with the comet's debris, so the Earth's atmosphere experiences a yearly collision with it, in mid-late October. Et cetera.}}

In the context of your example, the principal probably didn't need to know about anything more than the existence of the planned protest in order to suspend the students, so learn of and learn about probably indicate the same depth of knowledge.

So the shade of difference in this scenario is that learn about leaves room for an interpretation that the principal learned about a certain ingredient of the planned protest, that caused him to decide to suspend those involved.