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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There isn't a single word in English that would fit. Generally, "eager to learn" would be used.