I'm not sure I have a problem with the re-take actually. If the goal is for students to actually master content, why wouldn't you want them to retake it?
I'm a teacher and a school admin and we strongly encourage teachers to design their own tests based on what they actually taught, not the textbook test. Sometimes the whole class will miss a bunch of questions the teacher thought s/he had covered. They go back and cover it again and redo the test. That is a fault of the test and the teacher, not the student- so why wouldn't a redo be in order?
As for below 50%... well is there a difference between and F and an F-? My teachers would appreciate being able to stop at 50% and not have to keep grading down to a 39% or a 12% or whatever. If the line is "clearly you didn't understand this" there isn't much value in measuring how badly you didn't understand. So you need to go over the material again with that student and try a different way.
Also, my students only receive 0s when they have an unexcused absence and miss a test. Or if they just straight out refuse to do it. I work at a charter school that occasionally has students who do not want to go to this school and self-sabotage to get kicked out for academic reasons.
