We were doing a study to decide if we should divide the different educational related websites under different domain or subdomains (www.school.education.com) or all under one domain (different subfolders likeĀ www.education.com/schools ) or use the subdomain option for folder option for WPMU.
After a lot of study we decided to go with only one single domain for all the different resources (all related to education).

Let me tell you the reason,
We found that using different domains will divide our visitor and hence decrease the over all ranking of all our our domains in search engine.