You: For the same reason its opposite should be feigned.
But the "opposite" is not being feigned. We know that ERVs (and other genetic markers) are inherited. The pattern they display across species is consistent with the already-deduced family tree. So, this is simply one more bit of evidence to support the theory of common descent. See, no new hypothesis.
Introducing Occam's razor into biology class is introducing philosophy into biology class. As long as you're going to introduce it, why not ask which is simpler: To attribute cohesiveness and predictability to a single intelligent agent, or to attribute the same to random mutations, natural selection, and physical laws that came from unguided processes?
First, the Razor doesn't have to be in biology class; it's a general principle that applies to all sciences, and if it is discussed in class at all, it should be in that context.
Second, it's a bit unclear to me as to whether you're talking about biology or physics.
"cohesiveness and predictability". This sounds more like physics than biology, at least judging from what you said earlier
"physical laws that came from unguided processes". We don't know where physical laws "came from", or even if that is a sensible question. We do know that they have been observed.
To attribute ... to a single intelligent agent, or ... to random mutations, natural selection, and physical ...?
Clearly the latter is simpler. We know that there are mutations and selection, and also regularities in physics and chemistry. The theory is that these can account for all known biological phenomena. So far so good.
And I don't see what's simple in postulating an "intelligent agent" with (if it's the Christian God) infinite intelligence and power, or (if it's some other god or ET or whatever) with lots of intelligence and power, and then coming up with limitations on what it did (like make the biotic world look evolved), speculating on its "motives" (assuming that its intelligence is akin to human intelligence), and so forth. Seems like a lot more work than using what we already have.
What we know about this is exceedingly limited in view of the time and effort involved with observing it. Connections in morphology do not necessitate connections in history. It is not unreasonable in the least to assume them, but if you do, be advised that the evidence will fit every time. The deduced family tree of which you speak exists because one has already assumed (induced) a historic chain reaction that progresses from the simple to the complex. If one approaches science from such a standpoint it is a simple matter to "explain" all things without reference to a deity.
. . . the Razor doesn't have to be in biology class; it's a general principle that applies to all sciences . . .
It may or may not be applied in these disciplines. It is by no means a self-evident truth. Just another handy tool that may or may not have explanatory power. It would be a mistake to make Occam's Razor a test of what constitutes objective reality from case to case. There are times when a more complicated explanation not only fits in theory but also represents what is happening.
. . . it's a bit unclear to me as to whether you're talking about biology or physics.
It is even more unclear to me how one could treat the subject of biology without reference to physical laws. Same with chemistry. Maybe you can compartmentalize the sciences and maintain strict boundaries between them. I would think that to be tough and unnecessary. But you are free to accept and propound whatever kind of science you wish.
Public schools ought to provide classes and textbooks that accomodate atheistic, theistic, and agnostic science, for the students (whose parents' tax dollars support the schools to begin with) each begin with one of these sets of ground rules which in turn color the interpretation and explanation of the universe as we know it.
If it proves untenable or impractical to accommodate multiple starting points, then it is best to stay away from positive statements that conflict with them, or at least make the points with qualifying tone.
Science is loath to declare it has all the answers. It needs to keep that message at the fore.