Mary, thank you a lot for your help! I found that students were given permission to add news within the whole system. It is strange, but it was set by default. Anyway with your help I could fix this one.
But just an hour ago I encountered with another problem. Probably you also know the cause of it. Our studets reported us that they did not receive feedback comments for their completed assignments (assignment type: "uploading of a file"). I entered as a student and really could not find any feedback comments despite they already were written by teachers. The deadline of the assignment is 20th of January. Does it mean that student cannot receive feedback before this date or something is wrong with our system?