I've worked at various publications whose house rules forbade putting a comma after a conjunction if the result would be to isolate the conjunction either (a) at the beginning of a sentence, with a comma following it, or (b) at the beginning of a clause, with commas on both sides of it. Thus, in serene disregard of what one might argue is the more logical approach, those house style guidelines would require the following punctuation of your example:
This is slightly earlier than I'd propose for most students, but as we discussed, it is important to me that we have her well prepared for the October PSAT—a test that has little to no import to the average student—because of its national merit scholarship qualifying status.
and not the following punctuation:
This is slightly earlier than I'd propose for most students, but, as we discussed, it is important to me that we have her well prepared for the October PSAT—a test that has little to no import to the average student—because of its national merit scholarship qualifying status.
In (evasive) deference to those guidelines, I would, when possible, reword such sentences either as
This is slightly earlier than I'd propose for most students. But as we discussed, it is important to me that we have her well prepared for the October PSAT—a test that has little to no import to the average student—because of its national merit scholarship qualifying status.
(if the first clause seemed sufficiently independent of the following one) or as
This is slightly earlier than I'd propose for most students; but as we discussed, it is important to me that we have her well prepared for the October PSAT—a test that has little to no import to the average student—because of its national merit scholarship qualifying status.
(if it did not). An examination of punctuation conventions in the U.S. popular press should convince you that numerous publishing houses oppose isolating transitional conjunctions such as but and and with commas, even when a following parenthetical phrase might seem to justify it.
As for your question about whether you should "ever use a comma after a coordinating conjunction," my answer is that doing so is not objectively wrong—and indeed may be logically satisfying, since it sometimes helps neatly break out a parenthetical phrase (such as "as we discussed" in your example)—but that in U.S. practice it is by no means universally endorsed.
If you are writing to your own standard, use the punctuation you prefer; but be aware that some style guides strongly oppose adding a comma after a transitional conjunction.
