coollikethat52

http://www.gse.harvard.edu/admissions/live/class_profile.html

Masters are not as selective as Doctorates, and you can see these statistics on US News Rankings or even on some university's websites. Generally, Masters acceptance rate is anywhere from 30 - 60% of all applicants, while Doctorates are usually below 10%. But it depends on the school, and it depends on the department and field. For instance, Higher Education tends to be more competitive.
You can always call the department you're interested in and ask. They're very helpful and nice.
If you have questions, feel free to ask me.

I second this. As a current HGSE student (not in the Master's program, but I work with faculty members who are on the admissions boards), I can safely say that people in the Ed.M program come from all professional backgrounds and experiences, and this is especially true of the Psychology-focused programs. What is more important, as hitomimay states, is a clear interest and 'narrative' that is evident in your SOP. I would make your decision about what to major in dependent on your long-term (i.e. post grad-school) goals and interests, as they stand now.

Any other HGSE-related questions, please feel free to PM me!

Honestly, I think what major you choose is irrelevant. I was a Biology major before this. What matters is truly your statement of purpose, and your experience. Try to find professors who are in Education-related fields and see if you can volunteer in their research, or volunteer at a child development clinic and use these experiences in your SOP. Also a strong GPA and GRE score won't hurt. But long story short, major doesn't matter.