how do I do this
To access the Duolingo for Schools features head on over to your dashboard: https://dashboard.duolingo.com/
The Help section, linked to at the top of the page, answers a lot of questions and has a helpful step-by-step guide on how to add students to your Dashboard. Here is a direct link to that portion of the page: https://dashboard.duolingo.com/dashboard_help#how-do-i-get-my-students-started-on-duolingo
If you've already created classrooms instead of clicking "get started" click into the classroom you want to add students to and click the blue "invite students" button near the top of the page.