Parents can't remove previous teachers from their accounts. When a teacher archives their finished class, it removes the ability to communicate via Messages or view Reports. But, Story and Portfolio posts are actually designed to remain in your feed so that the memories can be kept there :). As your child's new teacher starts to post, the old posts will be pushed down so soon you'll only be seeing new posts at the top of your newsfeed.
That being said, you should no longer have an active connection on your account to a previous teacher. An active connection would mean that you could still message this teacher. If you are still seeing an active connection to a previous teacher on your account, please reach out to them and ask them to archive their former class.
If you are having any issues getting in touch with them, please contact us at firstname.lastname@example.org so we can assist you with this process!