My clan is going to do a LAN-Party and I wanted to know whether it is possible to get LoL into LAN.
nope, most of LoL's program is server-sided.
you'd need Riot to release their server program, and they're never going to do that.
...or you should reverse engineer that, wich is extremely hard (but possible, i know a few guys who did part of that) and forbidden by the ToS.
You'll need internet connection to log on their servers and play.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.