The title pretty much says it all - it's a 2.5 hr class and he thinks it's 2 hrs. The first 3 classes have been rushed and I want to tell him - but when I mention it to my classmates they tell me it's not our problem and adamately say not to mention it - I don't think anybody wants the class to be any longer than it is. What should i do?
Who said it was 2.5 hrs? The schedule? It is not uncommon for professors to reserve a longer time than is always necessary, just to give themselves a bit of lee-way, in case they sometimes need a bit longer. A 2.5 h time-slot it not necessarily a commitment to hold class for that long (but check your institutional rules if you feel cheated). The first class of a term is also often shorter.
The above is true at least for the Australian National University.
In any case, if you will ask him, it pays always to be polite and not to come across as though you are accusing him of intentional wrong-doing; but rather as if you are ignorant of their traditions.
I don't think anybody wants the class to be any longer than it is
Really? I would be surprised if this statement were true. If I paid to see a movie and the theater stopped showing the movie half an hour before it ends, I would be upset that I did not get the experience I was paying for.
Similarly, it seems reasonable to assume that possibly you (since you are asking the question), and almost certainly at least some of the students in the class, are interested in learning the material the class is meant to teach, and in getting the full experience rather than an abbreviated version. Even for students who are not interested, it’s wrong to say that it’s “not our problem”. It sounds completely conceivable that you will be tested on the full material of the class despite the fact that the professor will teach it in a rushed and difficult to understand manner (as you say has been happening in the first lectures) because of his misunderstanding about the lecture time. It’s also possible that once he finds out about the mistake later in the semester he will end up scheduling additional lectures to make up for the lost time, which would end up inconveniencing you and the other students and making your life more difficult than it needs to be.
As for what you should do, that’s for you to decide. But what would strike me as most impressive personally is if you Do The Right Thing (whatever you decide that is) based on your own convictions and not listen to peer pressure from your social group.