In the higher education it is not a lecturers job to convey the material to even the slowest of the class. The responsibility is entirely on the students side.

Education is selective and there is nothing wrong with it. Self-education is a very important part of it, and the only thing teacher really must consistently do is to help the students to identify their flaws as quickly as possible so they can go and address them.

But, yes, everyone must be able to retake any course any number of times, until either passed or pissed off.