This little post would have been absolutely charming if it had been written by someone whose first language wasn't English. It would have been adorable and would have made me smile and then lament that English has all sorts of crazy rules that cannot possibly make much sense to non-native speakers, such as when to use "good" as opposed to "well."
But this was written by a native English speaker. One who is clearly very confused about when to use "well" versus "good." If he'd really wanted to use the word "well," he could have said "He wasn't sure how well I'd learned English," and that would have worked. Or "He wasn't sure how well I spoke English" would have also been fine.
What he should have used in the sentence he wrote was the word good, as in: "He wasn't sure how good my English was..." But he didn't. (And yes, that sounds a little awkward, but it sounds a whole lot less awkward than using well there.)
If this person frequently made grammar errors like the one above, I'm really not surprised his teacher thought English wasn't his first language!