Just high enough that you can get into college. Honestly, most astronomy jobs require that you have a PhD. So after high school, you need to go to college for an undergraduate degree in Astronomy or a related field, and then go to grad school in Astronomy to get your PhD. Most of the people who have made it that far were probably good students back in high school. But when you're applying to grad school, no one is going to look back at your high school grades and reject you because your GPA was too low way back then.
Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.