Some servers require any connecting machine to have a reverse DNS entry - i.e., to be able to look up a host name for the given IP address. If this is the case with your school's software and you are using a broadband ISP (like Virgin) then only they (Virgin) can provide one. It's not something you can setup on your PC.
However, if you made your PPRuNe posting from that same PC then I can tell you that they are already providing a reverse lookup - I can lookup the IP address from which your post came without difficulty.
On this basis, assuming you're using the same PC, it isn't a reverse DNS problem.