I Want To Set Up A Reunion Website For My School. Where Can I Get The Cheapest Hosting?

Probably My space or Google. Go to technorati and check out how to set up a website or a blog where they can leave posts and comments.
You should be able to do this for free, unless you want to have payment facilities on the site, and secure payments for credit card transactions. However, as a place to gather and leave information the sites above should suffice for your purposes.
Friends Reunited have a huge site that was recently sold for a million pounds plus, but that was a business set up which required subscribers to pay a join up fee. Since it sounds like you just want to get together and share information this may not be necessary.
Many people use My space, and if neccessary, you could add adspace from Google, or register a domain that is offered and advertised on all these sites.
I hope your venture turns out to be a winner.