IMO, a forum could be much better off than creating a website. As there are free to use forum such as phpBB, what you need is just Apache + PHP + mySQL. And all these can be found in a package called xampp.
and install, next, download phpBB
(Download the stable version) and install. Now you have a forum where the girls will just have to register in the forum and use. They are able to do what you are doing now. Register, post topic, post reply and everything.
Now you have setup xampp and installed phpBB, next you will need to host your forum onto the net so that the girls will be able to access to your forum. No-IP
would be the best solution imo. You will need to register with No-IP, download No-IP. The No-IP program is for you to update your IP address with the server each time your IP changes. So that the free domain that you chose would be able to access. Read more in the website.
The above are all free as I said. Just my 2 cent suggestions. If you only want this forum to last for the summer, then I feel that it isn't worth it to pay for it if you are only using it temporary. The only down side I feel is that you might need to run your computer 24/7 so that at any point of time, the girls are able to visit the forum.