I tell my students that if you're going to do something, you may as well do it right. I also ask them if they want to sound like a kid banging on the piano, or do they want to sound good?

Have your other students expressed the feelings of the student who left? If not and they are happy with your high standards, don't worry about it. You can't make every student happy. Remember, we are not going to be a good fit for every single student.
For example, a mother called and asked me to teach her son so that when he got to band he would be ahead and maybe sit first chair. (Some how I got a reputation that my piano students do well in band.) She wasn't interested in festivals and all the other things I do. I told her she really needed a another teacher because I want long term students who are serious about the piano, even if they sign up for band, too. At least she was honest.
Many high school students want to play well, and appreciate your high standards even if they don't want to major in music. We need teachers like you for them!