I think this can be read three ways, because constructions like this are used in different circumstances:
I haven't seen such a big fish today. Until that one. We haven't seen such a wet winter since records began. Until last winter.
Here the time period is arbitrary. I happen to be talking about today, or about recorded weather. I'm not saying the last winter before records began was wetter or drier than last winter. I'm not saying if I didn't or didn't see a bigger fish yesterday. I'm just not talking about that long ago. "Such" clearly refers to fish/winter that I introduce in the sentence fragment, not to anything that happened exactly at the start of the time period I'm talking about. So here I'm not making a comparison.
I believe we can rule this interpretation out, unless we wish to be way more pedantic than it's likely the author intended us to be. The author might in theory have selected slavery as a completely arbitrary time period, but since slavery is itself an exploitation of physical abilities, that would be some coincidence. No, the choice isn't arbitrary.
We haven't seen such a genocide since the Second World War. Until
(Please don't debate how bad a genocide Rwanda was or talk about the Khmer Rouge or whatever: my point here is not about whether my hastily-chosen example is correct, just the construction of the sentences).
Here, the Holocaust is an extreme case of genocide, and we've chosen the Second World War to mark off our time period because of that. We're not saying here that Rwanda is worse than the Holocaust. We're saying that it's the worst in the time period 1945-1994. "Such" again means "as much as the thing I'm about to mention next", not "as much as the thing I'm using to limit my time period". I think this is probably how your teacher interprets the quotation.
And although we're not quite saying that the Holocaust was worse than Rwanda, we're strongly suggesting that it is worse or else why choose it to mark off the time period we're going to consider, a time period that by design excludes the Holocaust? So we are making a comparison between the two, and we're saying the Holocaust is worse, and that's why we've chosen the time period "since then".
It is worth bearing in mind that when someone says "this is the worst since X", and it's possible to compare X with this, then they do usually mean that X was worse than this, so they are making a comparison. Sometimes this leads to unintentional humour when they don't mean X was worse: "9/11 was the worst thing that had happened to the USA since Bush was elected".
Still, I'm not entirely satisfied with that interpretation for two reasons. Firstly because of the way the quotation uses the full stop and the sentence fragment. This abruptness suggests to me that the author actually intends it as a counter-point of what went before, "It used to be the case that slavery was the worst we'd seen. Then college football happened". But it's not conclusive and it might not be intended that way just as my Rwanda example isn't intended that way. Even if it is intended that way it might be intended as hyperbole, it's not possible to tell without context. The second reason is that if "such" is supposed to refer to "college football", off in another sentence, then it's much clumsier than if "such" is supposed to refer to slavery.
So, I can't rule that interpretation definitely right or wrong. It could instead be interpreted with "such" referring to slavery. Then it becomes like:
Never had we seen an exploitation of physical abilities as bad as
slavery. Until college football
That would be unambiguous and would be a clear comparison of college football with slavery, saying that it's at least as bad an exploitation of physical abilities. Against this interpretation, the author could have just put it this way, and didn't. Other people have compared college football to slavery more directly, but this author chose to use a construction that could reasonably be interpreted not as a comparison between football and slavery, but only as a comparison between football and anything that has happened since slavery.
Lacking any other context, I'm inclined to take the quotation as being deliberately vague about exactly what comparison is being made, and even whether a comparison is being made. The comparison is insinuated but not exactly stated. Whatever order we place them, we're invited to hold in our minds slavery and college football as the two big exploitations. Your teacher's interpretation avoids guessing what the author is trying to make us think, and sticks only to what is definitely said. But I still think your teacher should have taken into account that even in my middle interpretation, there is an implied comparison between the two.