Question: What adjective (single word) means 'having/using a willingness to learn'?

Answer: 
I understand you're looking for a single adjective, and I think "curious" fits the bill nicely (and from a pedagogical perspective curiosity is a key indicator of learner success) but I would like to add the following to the mix because I think it shows, if not willingness, at least readiness or capability, and it might be worth something to you:

**fertile mind**

From Collins:

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> *fertile mind:* One that is capable of sustaining, supporting, growing and developing mental thought interjection, especially among the very young.
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Having a willingness and capacity to learn is **teachability**: 

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> teachable adj.
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> 1. Capable of being taught: teachable skills.
> 2. Able and willing to learn: teachable youngsters.
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> teach′a·bil′i·ty, teach′a·ble·ness n. teach′a·bly adv.
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> American Heritage® Dictionary of the English Language, Fifth Edition.
>  Copyright © 2011 by Houghton Mifflin Harcourt Publishing Company.
>  Published by Houghton Mifflin Harcourt Publishing Company. All rights
>  reserved.
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