A teacher can create a Google Community within Google+. Privacy levels can be set so that only members of the Community can see the content and notifications can be turned on when posts or comments are added to the discussion.
Students will need to create a Google account if they do not already have one. Since we are at the high school level, students meet the 13 year old age requirement that Google has in place for creating an account.
To set up a Google Community follow the directions below. (*Note the instructions are based on the "new" google+ layout not the classic.)