"But in the other course this was different!" - How to react on this if the other professor did not follow the rules? The situation

I'm a PhD student and serve as an assistant to several courses. Now, the students were complaining that some rules of our course are not good for them - there are some restriction what has to be achieved in order to participate in the final exam, e.g.


they need a certain amount of credits in their homework,
everybody who has failed that course in the years before, have the get the credits for their homework again.


You have freedom in chosing the threshold, but the rules are written in the examination regulations (I'm also a member of the that examination committee) that is not the problem. 

There are some professors in my department who don't follow that rules; or (even) worse come up with some funny new rules contradicting the rules (like (you don't have to read this to answer the question)  If you have 50% of the credits until the third to last week of the semester, you are able to participate in the final exam. If you pass, you pass. If you fail, you are only allowed to participate in the repetition of the final exam if you have 50% of your credits in the last week of the semester).

The problem

Apart from them violating the rules, the problem is now that students come to my course and say: "Hey, professor XX did not have the rules last year. Why do you have them? That is so unfair! And you can have it, too! Because, nobody complained last year! Cmon, we will not tell anyone!".

As soon as I hear something like that, I complain. The problem is then: No student would tell someone things like that (exept in the mentioned case when they try to convince me to do it that way, too); the professors are sometimes not informed, but also sometimes aware of that und don't write anything on their course homepage. So nobody can find out. If you find out (probably at the end of the semester), it is too late and if you want to declare that course as not following the rules, you are only damaging the students of that course. 

I was talking to the responsible persons in my department; they see the problem, but they don't want to have fights within the department and they don't to establish automatic announcements from the professors what the rules for their course are. 

The question

What can I do against it? The atmosphere in my course was very bad because of that. I could go to the head of the faculty or the university who will end all that, but then I fair to be an abandoned person in my department. Also, I want not to damage students and declare their passed exams as illegal.
This is a standard problem in places where a mentality of unwritten rules has become standard (you know, the places that have "rules" and actual rules). If this becomes ingrained in the university culture (both in the student body and the faculty), nobody can really tell anymore what is a an actual rule and what isn't (and, really, the answer will be different for each teacher). For this reason I prefer a relatively strict no exceptions policy in my courses. I understand the downsides of this model (see for instance also here: How do I appropriately penalize late projects?), but at least everybody knows where he stands in such a model.

I understand that you are not in a position to change university culture, so you need to play the hand you are dealt. I see two main ways to go forward:


Become more lenient yourself. If a number of senior professors deems it fit to not enforce the official rules or implement weaker rules, then maybe the official rules are in fact perceived too strict. In that case, maybe the answer is just to run with the herd and allow for more exceptions etc. yourself. Of course the preferred solution in that case would be to change to official rules, but it is well conceivable that this is not feasible for one reason or another.
Announce very, very clearly at the beginning of the semester that in your case the rules will be the rules, no matter how things were in previous years or with other teachers. Follow through on your claim, and build up a reputation as being a strict guy. Note that being strict is not the same as being an unpopular / bad teacher. I have continuously followed this strategy in my teaching, and still receive good to excellent teaching evaluations.


Of course, if you follow the second strategy, some complaining, accusing and guilt-tripping  will happen. It is your task as a teacher to be able to distinguish between warranted complains (and react to them) and more manipulative ones (that are to be ignored). These examples you give quite clearly fall into the second category. What you need to do is find a number of polite standard answers to these statements, and then forget them. Examples:


  Hey, professor XX did not have the rules last year. Why do you have them? That is so unfair!


Answer: sorry, but it is impossible to change anything in a course if I am forced to always do things exactly like the were done in previous years. I am trying to be more strict this year because I have seen that unclear rules are also not beneficial for many students, as nobody really knows anymore what the actual rules are.


  Cmon, we will not tell anyone!


Answer: while I appreciate that, I am afraid that I do not want to make an exception here. Making an exception for you is unfair towards the other students that did not get this exception.

Some other minor comments:


  I could go to the head of the faculty or the university who will end all that


I am unsure if that is even true. In all places that I have been to, it would be very unlikely that a head of faculty micro-manages how professors do their teaching.


  Also, I want not to damage students and declare their passed exams as illegal.


This is understandable and correct. If you truely think that students were under the impression that a certain rule or exception would apply to them, I would let them pass as part of a transition period. If you think that they were aware that they should actually fail, but simply try to come up with a reason why this is not the case, then be strict and let them fail. Note that this is the reason why announcing your strict rules enforcement as clearly as possible is key. In your situation, being strict is only fair if you give your students advance warning, as the seem to be trained by your university that, by and large, teachers are lenient.