Stealth means it can't get proper responses back from your computer, probably due to a firewall interfering with the testing. But if you can downld then those ports must be open.

I think the issue here is "school". Many schools block p2p sharing nowadays. And those that don't will often use a special program that shapes the bandwidth ... giving priority to browsing & low priority to other purposes. There's probably not a lot you can do about it. Bandwidth for schools costs them a lot of money & they want to ensure it is used for what it's intended for. Browsing & researching, etc.