Can't install El Capitan during Clean Install

I created a Bootable 16GB USB Flash Drive with Mac OS X El Capitan, and i've used it several times on different MacBooks, both on Airs and on Pros.

We use Office 365 and in order to use 2016 edition, i need atleast El Capitan.

What i normally do is hold Option + Command + R + P to reset the Param first.

Then i restart the MacBook and hold Option key again to be able to boot from the USB.

Then i Exit the installer and enter the Disk Utility and erase the HDD so it's empty.

I choose Mac Journal non-Case sensitive Extended partiotion Map, and everything seems fine.
I Verify the HDD to check if there is any errors, which there is not.

Then i Exit the Disk Utility and the El Capitan Installer window pops up, and i start the installation.

After about 3 hours, i get the message:

"OS X El Capitan was not downloaded, Go to Purchase-Page to try again"

I can't do that, since i erased the HDD, thereby there is no way that i can enter AppStore and retry on that MacBook Pro...

I've been googeling for 3 days in a row trying to find a soultiuon that might work, but nothing works.

I've also tried Recovery, which gives me the same error over and over again...

There is no Time-Machine backup, insted i've copied alle the user data that i wanted to keep over to a new external HDD.

Is there anything i can do to solve this problem?

This MacBook Pro belongs to a student at a HighSchool in Bergen, Norway.

I work there as an ICT-Apprentice, and i've never seen this problem before.

I've also tried to go back to Previous OS, which was Mavericks, but after the installation finishes, it reboots directly into the window where you can either choose between Time-Machine Backups, Use Safari to get help, Install Mac OS X again and Disk Utility.... (Can't remember what that windows is called).

I am seriously out of ideas and i'm about to rip my head of so i may never ever again have to do anymore IT Support...

PS.

I've tried the USB on both my MacBook Air, and on another students MacBook Air and Pro, and it works flawlessly on those machines, so i know that the USB is not faulty or corrupted...