Channeling shield

I think this is a rather niche question, but does the high magic ability Channeling Shield function only with the school of high magic its purchased from or can it be used for both schools? In other words, can you use Earth channeling if it's purchased with Celestial high magic and vice versa?

I thought that it would only work for the school it's purchased from but the CMA won't let me prepare it for both schools and I couldn't find it explicitly stated in the rulebook, so I thought that I'd ask
 
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