not all students progress is showing in the classroom.
You can't add other Duolingo accounts without their permission. Here's your best option:
If you go to your classroom and click invite students, and then share classroom link, it will give you a classroom code.
If your students go to the progress sharing part of their settings, there's a link there that will let them enter a classroom code. If they enter the classroom code from your classroom, it will automatically join them to it.