>> 256 MB RAM
> This is on the low side. At todays RAM prices there is no reason to save money here. For a responsive system (on Linux) I would advice to go for 512 MB RAM.
Only later I saw that you are talking about a shared server this difference could upset your budget ;-(
I don't have experience with this sort of servers, somebody else has to jump in.
My rough guess is that you hava a small site, 256 MB might be OK if a) the server is a fairely tuned Linux system b) your 15 concurrent users don't want to run a chat session in parallel.
BTW, do you plan to give those 15 users an on-line question paper (quiz) to be solved at the same time (say day X from 8-9h)?
You must have noticed that the _peak_ demand counts. If you can figure out what that is, we can give a better answer.