Is there a way I can increase the difficulty of the class activities?
do you mean the tested feature allows to enable to the student a skill even if they didn't completed all skills before that one?
If so, what my class activities page should look like if I have it? Or (maybe simplier), what is the name of the A/B test so I can in which group I am. ;)
No, class activities have no impact whatsoever on students (or on the teacher's tree, for that matter).
This test only affects the "power practice" session. If your description for "power practice" says "Pick any skill from Duolingo's curriculum to practice in class.", you are in the experimental condition. :)