It's not a matter of teaching religion. It's a matter of teaching religion in science classes and passing it off as science. Sure, not all schools would adopt such a ridiculous policy, but it would still leave the students at those schools that do at an educational disadvantage. Think "backwater."
There are tons and tons of schools out there that leave students at an educational disadvantage as things are now, because of poor teachers protected by unions and/or schools where no one wants to teach because they are too dangerous, etc. All of this is under a Federally managed education department.
It simply cannot get any worse and will probably get a whole lot better if local taxpayers are more in control of the choices they have, because local politicians are responsible for education.
The federal department of education has a whole long list of failures, including such winning policy choices as "No Child Left Behind" and "Common Core".
Carter created the department, and it was essentially done so as a payoff to the unions for their endorsement. Since then it's been one disaster after another.
Lastly, of the thirty or so powers delegated to the Federal Government in the constitution, education is not one of them.
Here is a nice chart, showing spending per pupil increases since the DoE was created, and what we've gotten for our money.
You asked for examples of waste, I give you the Department of Education.