Is there a way to get a class set up if the students already have an account? how do i do this?
Sure -- if your students already have an account, and they aren't being monitored by another teacher, you can add them to your Dashboard pretty easily.
The most common way is creating a new classroom on your Dashboard and then clicking on it, or clicking into an already created classroom if you have one, and then clicking on the blue "invite students" (or green "invite more students" if there are already students assigned to that class) button in the upper right portion of the screen. You should get a pop-up that displays a link to give to your students that they then use to join your class. If you use Google Classroom information about that is also displayed on that pop-up.
A second option is that a student can manually add themselves to your class in their progress sharing settings ( https://www.duolingo.com/settings/observer ) if you give them the email address associated with your account and the name of the class/section they should be in, if applicable.
If you need more detailed instructions with images to help you along, here is the relevant Help section related to your question: https://dashboard.duolingo.com/dashboard_help#how-do-i-get-my-students-started-on-duolingo