Accounts on are completely separate from those in what we call School & District Accounts ( for example). So you'll need to create new accounts (either via import or using the web interface) for any teachers that you want to have access to 

If they already have classes on that they want copied over to the your domain please contact us with their usernames & we can take care of that. In the end, those teachers will have two accounts & it's up to them to decide whether they want to keep their account for personal use or not.