My own Catholic school experience is that theistic evolution is taught in religion class, and plain ol’ evolution is taught in biology class. Religion is religion and science is science.
I grew up in Pennsylvania, and all our Catholic school textbooks except religion books were provided by our local Intermediate Unit, which means they were paid for with taxpayer money. The same titles were used for public and non-public schools. So none of our textbooks had a religious slant except for textbooks specifically used in religion classes.
We were mostly taught that God set things in motion (evolution), but that’s about it.
And when my first grade teacher told us about Adam and Eve, she didn’t tell us it took a week to make the Earth, but to make the Garden of Eden, (basically, God had to clean up a nice spot for them, because it was mostly muddy and such). 
And she was a nun.
This is where a lot of the problem comes in. Evolution isn’t a theory. It’s a fact. What confuses people is the title “DARWIN’S Theory Of Evolution.”
There are many theories that vary to how evolution came about, Darwin’s theory is just one of many. The theory part applies to Darwin’s slant on it, not to evolution itself
This is one of the bad things that is passed off in science classes - ie, that science is concerned with ‘absolute truths’. Further, I suspect you are under the notion that evolution should be something other then a ‘theory’?
If so, then you are pointing out another problem with science education - that people aren’t being taught the proper vernacular of science. In science, the term theory denotes an explanation of a wide array of facts and laws - it does not denote uncertainty.
The germ theory of disease explains how germs get people sick. No one seriously doubts this, nor do they doubt heliocentric theory or the theory of relativity.
My guess would be that this is probably due to inadequate education on the matter.