As high as possible in the right courses. My survey of astronomers shows that in physics and math courses, astronomers got very high grades usually above B+ in all courses. Grades such as 'A' or 'A-' are probably quite common in most astronomer's undergraduate courses. In graduate school, these grades might slump a little to 'B+' and 'A-' mostly, because you are now competing with all the other 'geniuses' from around the country that enrolled in your schools graduate program. I know of no astronomers who admit to getting grades in the 'C' to 'B' range, and it wouldn't make sense if there were many of these in the profession. Astronomy is very competitive, and there are plenty of very bright, straight-A students looking to get into a career.