Here is a link to a forum ran by MIT. When you post a thread it's instantly receives a PR of 8. Don't Believe me? Go try it!
Toolbar PR is updated every few months, so this is incorrect. If you're using a plugin to view the PR of the new thread, it's probably showing you the homepage PR instead of the PR of the new thread. Use a different PR checker to double check the PR of the new thread.
unless there is a post that gains rank quickly - i dont see how anyone can get a pr8 link without being on the main page
Posting in a forum for the sole reason that its homepage has a high pagerank is going to be a waste of your own time, and if you're spamming the forum, it'll just get you banned. If you want to use forums to promote your website, locate the forums that are most directly relevant to your website's topic. Participate in these forums over time, build up a reputation, and add signature links if allowed, and post relevant links to specific pages on your site, if allowed. I've seen decent results with that approach. Honestly though? I've found that overall, forums are a poor way to promote a website. Relative to the amount of effort you need to put into them, the links in them tend to drive little traffic, in comparison to carefully writing articles and submitting them to article directories, or working on more intelligent or sophisticated ways of promoting your website. Even the best forums tend to have limited readership, and the lifetime of a given thread is rather short: people read it briefly and then it is quickly buried. There are so many more effective ways to do internet marketing.
There are better ways to get .EDU links than spamming in a forum, those will get deleted right quick.