Installer gives error message saying the package couldn't be opened, and that the certificate wasn't valid. why?

The download for the installer took about an hour, with the download time fluctuating wildly from one hour (about a minute before it finished) to three hours to even two days. Right before it “finished,” the progress bar was at maybe 2% and it said it had 1.7 MB of 35 or so MB downloaded. What the heck is going on? I’m using a school computer so that may have something to do with it. They’re using something to screw with the internet connection here, I’m sure of it. Even though the principal himself has used his passkey to get through it, nothing seems to have improved. Help?