I've been looking for MPH's salaries online, but I can't seem to find salaries based on school. Does attendance at a school of more prestige (ex. GW) affect future salary?
Depends on what you're planning to do with your degree. If you plan on stopping at an MPH and work in a local public health organization or work as a research project coordinator, the reputation of your school won't matter so much. If you plan on getting a PhD and you want to lead research at a top university or one of the big government organizations, like the CDC, then reputation is much more of a concern.
The school by itself won't dictate your salary. What kind of work you do does. You need to go to the school that is the best fit for what you want to accomplish.