Hi! You may be interested in my post about textual criticism here.

There are no surviving originals in the manuscript tradition (the technical term is "autograph"). Everything we have, outside papyri, inscriptions, and odd things like the Vindolanda Tablets, is a copy of a copy. For Plutarch's Lives, our earliest manuscript is 10th C.

As for his use in medieval education: I'm not sure about that. In Brill's Companion to Plutarch, Pade writes:

In the Latin West probably no genuine writings of Plutarch circulated in any form during the Middle ages....

Which makes a lot of sense, considering the near total loss of Greek in the West.

A little later:

So, by the middle of the fourteenth century Plutarch was known in the Latin West almost exclusively as the author of the Institutio Traiani, and accordingly as the teacher of Trajan.

That's falsely attributed to Plutarch if not an outright forgery, and in Latin.

Still a little later:

even if Plutarch’s name was not completely unknown in the Latin West by the second half of the fourteenth century, no genuine work of his was available to Latin readers until the Greek scholar Simon Atumanus translated De cohibenda ira (On Restraining Anger) at the beginning of the 1370s....

The Lives were translated only 10 years after that. The first printed Greek edition was 1517.

Since Plutarch was almost entirely unknown in the West, which I at least usually associate with the term "middle ages," it's hard to imagine how he could have been used in education, and I'm interested to see the source of that claim.