If the URL you had initially selected is not Grade specific eg. jbennett.commons.hwdsb.on.ca and not jbennettgrade1.commons.hwdsb.on.ca then you can just go in and delete all the posts.
If you would rather start fresh, we can delete your old site and create a new fresh blog for you to play with. Alternatively, you can delete most of the content, leaving the customizations to the sidebar, or to the additional more static pages, and we can change the URL for you. Check out the A Good URL article for some tips on selecting a URL that will stand both the test of time, and the occasionally transient nature of the teaching profession (different schools, different grade assignments, etc.).
Contact your 21st Century Learning Consultant to help you with these different options.