is it true that once you start working, the real world is really dark?

It doesn't have much to do with what job you choose, it's more about having responsibilities. Paying bills, taxes, student loans, etc and having to balance work with personal relationships and a social life.
That's why people tell kids to enjoy their youth because once they're living in the real world, things get complicated and rough.
For example you can't just blow off work like you could skip a day of school. Because your whole life depends on that job, not getting fired and being able to maintain a household.

Every work environment is different. It largely depends on the Industry you're in, and the Culture that Upper management have instilled in their workers across all levels. There are plenty of Employers that employ based less on Academic results and more on how you treat people. The problem with most work environments is that you simply cannot know how good or bad they are, until you're working there.

No, it's not true. It's entirely dependent on what you do for a living, where you do it, and who you work with.