Are you sure that the ports you can use from the intranet of your school are availble anytime from the outside? Are you not, then you need a portscan to verify it. But here i warn you, i do not know, if a portscan from a host your a not the owner is legal in your country!!!
Reason that it not able to connect from the outside ist that your school using port triggering, that means that only if there an active service internal the port to external sources will be open, means if you try to connect to Blizzard server about port 6112 it is open, if you are not it will be still closed. If your school is using port triggering forget it, because a service like dchub is passiv and waiting for active connection from the outside. Therefore ftp need port forwarding as well.