Multiple usernames & passwords for one page on my site

just wondered if anyone can help me out on something :

For the site https://www.dramaticmoments.co.uk I am trying to make a password protected page, whereby multiple passwords and user names can be set up (around 100) for the following page:

https://www.dramaticmoments.co.uk/index.php/the-course

At the moment there is only one, and this doesnt prove to be very safe. Is there a decent plugin????

Also wanted to find a plugin to create a forum/competition where students can log on and write an answer to a question that we will ask. I would like it to work like the iphone text conversations
(https://gadgetcrunch.net/wp-content/uploads/2008/02/iphone_text_message.jpg) Do you know of one????

Please let me know if you can help with either of these. Cheers.