Teaching C#, Java, etc. to students who are bad at OOP
I know exactly what you are talking about. Back when I was still ( I am glad I am no longer doing that) working as a faculty in a university, I ran into situations like this. In fact, with my usual style, I got confrontational with the faculty who was supposed to teach OOP in the previous semester and stuff happened.

Here is what I did. 


First up, the logistics. I had a detailed discussion with the HOD and also with the students, and convinced all of them to come for additional classes (I called them Bootcamps)
The Bootcamp was a 2 day affair ( 6 hours each ) and was held on a holiday. No attendance or anything like that. Students who are serious can come (and 80 % of the class showed up)
Taught the basics of OOPS. I essentially summarised the entire semester worth of contents into 12 hours. 


Did this work? For me, it did. I was able to carry on with my usual syllabus with confidence that they are at least, not as blind as before. sure, I had to spend 2 of my personal days, but it was worth it. 

Here are the things I crunched on those 2 days.


Classes and Objects (and the interconnection between, and compared them favourably with structs)
Encapsulation (especially private and public and why this is important)
Inheritance (the big boss of OOP, and by far the most important thing about any OOP language)
Polymorphism (although this was pretty straightforward and did not drive people crazy like inheritance or the initial discussion of classes and objects)


So, talking from first experience, the situation you described is the norm rather than the exception. If you want to get them up to speed, I suggest a bootcamp or a similar variation.