QuadBlogging is a site that help classroom blogs connect to each other :
Four schools have a partnership/agreement for a four week cycle, each school’s blog would be the focus for one week out of four. Each school in the Quad would spend some time visiting the blog of the school for that week, leave comments etc. After that week, another one of the four schools would be the focus and this would be repeated for the four week cycle and then repeated. It wouldn’t take the pupils long to work out that during their week, they would get a boost in visitor numbers and comments. This would give a real focus to have posts online ready for this bulge in visits. During the other three weeks, pupils get to visit and comment on other blogs in their quad. Pupils being pupils, they would also venture out of the quads and visit other blogs that are linked.
There is a signup form on the QuadBlogging home page.