filter just one table by login

From you description I think you need a separate security table: Two columns Login and Class which is related to your Class table based on class.

Apply the RLS to this security table which will then filter the class table to just show the correct classes but will leave you able to see the other students in the class. You might have to play a little with filter direction between existing class and existing student tables. If you can build a demo pbix file I can help futher.