How to set high class expectations and still maintain a good class relationship
Let me second the post of Massimo Ortolano and especially point 2. there. But you can do more. 

One way to keep students happy, both the really hard workers and the strugglers, is to give them a second chance when they fail to live up to standards. One way to do that is to permit students to resubmit work with corrections after grading. My experience is that you can be as demanding as you want to be as long as students don't feel trapped or disadvantaged by what they see as small errors. 

I've permitted students to resubmit work even after I've discussed the correct solutions in class. Just going through the motions of typing out the correct answers is an educational experience - reinforcement. I've usually permitted more than one resubmission of the same work. You need to set a limit, though, if students neglect new assignments to re-do old ones. A bit of balance is needed. 

To be fair to those who get it right, it is also useful to extract a bit of tax on the re-work. Say, only 90% of the difference between the first marks and the full marks can be achieved through rework. This also keeps people from being lazy about deadlines. 

Note, of course, that regrading requires that the number of students not be excessive and that you develop efficient ways to see what has been changed when re-work is submitted. I just had students submitting new work also submit the old (with my comments) as well as highlighting changes. 



But with respect to the point of students needing a while to learn to live up to new and higher standards, I also experienced this when I brought new and higher standards to a new job. Students in a Masters program had gotten the idea that good grades were pretty automatic. They were a bit shocked when I told them that it was possible for all to fail and that I would read and comment on all of their work. It required backup from the Dean, actually. But it didn't take too long in my case. Word spread quickly.