I was never enrolled in a CELTA Program but I do have many years as a teacher of adults so what you are relating is a familiar dynamic between a teacher and students.
In general, no matter what adult classroom you refer there are, at times, going to be "issues" relating to the interaction between teacher and student.
It seems in this particular case the teacher may have felt inferior when dealing with knowledgeable students in the class and so may have either, consciously or subconsciously, treated some students differently.
The most important element at the end of the day is to ask whether the teacher was fair. Did everyone who put in a reasonable academic effort become successful?
If so, don't let the actions of one teacher colour your perception of classroom dynamics. As a potential TEFL teacher you have learned more than a language learning lesson.