I often recommend the use of the Sitewide tags plugin as a means to pull in all the posts from all the sites across the network to the front page, or a secondary blog.
Even though the plugin is pretty straightforward, a lot of people get tripped up on the little details.
Installation: Install the plugin file in the mu-plugins folder please. This way it works across the entire network and is immediately working. If not, you can run it from the plugins folder but it *must* be network activated. Read more about the mu-plugins folder here.
Scroll down and tick the checkbox to tell the plugin to start gathering new posts.
New posts will be added as they are published to the public sites. If the privacy setting is set to anything else, these posts will not pull through to the tags (or main) blog. You can find the privacy setting under each site’s Settings -> Privacy menu.
For a working example, visit the home page of HomeschoolJournal.net.