The Edublogs guide to obtaining permission to blog with students includes a sample permission slips that you can copy and modify. The guide also includes framework for having discussions with school administrators and with parents about why you want your students to publish their work. Another good component of the guide is a set of guidelines for students and parents regarding publishing and commenting behaviors.
It's guides like this one and other quality support resources that helps keep Edublogs at the top of my list of recommendations for student blogging platforms. If you're interested in getting your students blogging or using blogs as digital portfolios, take a look at the directions in my Practical Ed Tech Handbook and then create your first blog with Edublogs, Blogger, or Weebly for Education.