Hi @German Dolnikov.  I wanted to start by pointing out that if you are a teacher for an accredited educational institution, you could apply for the Miro Education Plan which is a free, 100-seat plan that has (mostly) the same features as the Team Plan. While this would only give you and 99 students a paid seat, it still is not required for you to accomplish the goal you have outlined.

My goal:
1. I have about 300 students. They only need to view access (no need to edit)
2. It is important for me that the board does not go into free access to the Internet and the student cannot clone it for himself

The Team Plan at $10/month (or $8/month if paid annually). With this plan, you will have access both the Board Content Settings and Guests (as Viewers or Commenters only) features. 

The Board Content Settings will let you configure the board’s Sharing permissions so that only you, the board owner could copy board content or duplicate the board.

The Guests feature will allow you to add your students as viewers (or commenters) of the board, without having to make the board public.

To see this in action, I will add you as a viewer of a board that I created in a Team Plan that I am a member of.