Is there any way to split students into two or three groups per class? I don't want to create different classes for students in the same class. I would prefer to assign, for example, 20xp to 4 students and 50xp to the rest of the students in the same class. this would greatly assist when I have to modify lessons for lower learning students or assign more work for higher learners. Google Classroom offers this option.
No, sadly that's not possible. You can only assign tasks to whole classes. Students can be in more than one classroom at once, though, so you could have a student who was in e.g. Yr_8_Spanish and Yr_8_Spanish_top_group.
It's also not possible to share a classroom between teachers, or hand a class over to another teacher, or to import users by uploading a CSV file, etc etc.
There's a lot of stuff that a teacher needs that hasn't been implemented yet - hopefully Duolingo will fix this stuff before they aim for wider adoption of Duolingo in schools.