When is it acceptable to report classmates who cheat on an exam? So for one of my computer science classes, we have a WhatsApp chat among all students in the class to network and help each other out with questions regarding homework, or if we have any confusion on assignments. It's a given that we can get help on these types of assignments. 

But exams are proctored online, and getting help on them is strictly forbidden. However, some students in the chat have posted to the WhatsApp chat during their proctored exams, posting photos of the exams, the questions, and receiving help from others during their exams.

Why would I care about this? So normally, when a student cheats on an exam, it would have no effect on another student because the two overall students' grades are unrelated. However, the exams are curved. So these students that cheated on the exams **not only have an unfair advantage, ** but they also put other students at a disadvantage. Them doing exceptionally better than they would have has the potentiality to reduce other students grades by up to 10-15% of what it would be. So if Student A would normally get a 65 because they don't know the content, they now get a 89, thus, throwing off the entire curve. However, student B who actually DID try his hardest and studied got an 74. Their grade would have been higher with the curve, but student A cheating disrupted the curve. As a result, student B gets an 79, where they would've had an 84 without the several cheaters with almost perfect exam scores at the top.

Personally, I hate to be 'that girl' that reports someone else like a snitch... But I worked so hard in this class. I studied for 6 hours every day before this exam, and got an 83... and these people admit in chat that they would probably fail if they didn't use WhatsApp (have it screenshotted-saying that exactly). So I worked so hard and got an 83. They cheated and got a solid 90, doing no studying, no hard work on their own part. They work full time, so they complain they don't have time. But how is that my fault, or the fault of the other students that the cheater makes different life choices, or chooses to put less time into school, and we as a result should suffer? That's my rationale for wanting to report them. I play fair, so why should we suffer because they don't?
The answer to the title question is: 

Always 

(barring some sort of really bizarre circumstances that I am not able to think of right now).
The specifics of the situations does not make me change my answer in this case.