Yes: because comments should not be off-topic, inappropriate, offensive, hateful, or profanity-ridden. However, this means the blog administrator is required to monitor all comments frequently, or else risk not approving and posting the appropriate comments and thus stifling/slowing down the conversation.
No: because students need to learn how to write and publish responsibly for a wide, authentic, online audience. A school culture should be created in which off-topic, inappropriate, offensive, hateful, or profanity-ridden comments are not accepted by peers and the offensive poster is seen as ridiculous (and thus naturally “shamed” into changing his/her behavior). In this case, inappropriate comments can be deleted by a blog administrator as they come up.