Thank you for suggestion, Glenys! I will try ForumNG module to make new forums. But I am still looking for a solution to delete all student posts (i. e. store posts in another location - may be a hidden forum - I don`t want to delete them as they may be needed in the future) so that the forum could be ready for new students.
Also I think It is possible to create a new forum, copy and paste teacher`s post in that new forum and then make the old one hidden. As you can see, I think I have just found an answer