If the class average is too low, do profs usually curve or adjust the grading scale?
When it comes to grading, there really is nothing that can be considered "normal." As for adjusting grades, the answer is a definite maybe. It really depends on the instructor. I do, but many of my colleagues don't, but then they give tests with much higher averages than I do.

Exam grades and difficulties can vary from course to course and from year to year within a course, let alone between departments and institutions. There will generally be an attempt to bring grades in line with "expectations" and requirements of the school. But what those expectations are can vary widely. For instance, at one university I worked for, there was nothing wrong if a third of the students or more failed; at my current institution, it would create major headaches if so many students failed.