Solution 1:

I haven't used Agrarian Skies, I assume you can't just enter a world seed into the normal seed box when creating a new world?

If not, you're left with one of two options:

  1. If you already have a world with the correct seed, then use a utility like MCEdit to bulk clear out all blocks.
  2. If you have a world that you want to use, but it doesn't have the right seed, you can use a utility like NBTExplorer to view and edit the save file, and change the world seed.

Solution 2:

When you're starting a Minecraft multiplayer server, you can add a setting to your server.properties file called level-seed: <data> to force a predetermined seed to be used regardless of world settings.

Watch out, though!

  • If you remove the level-seed tag, then the server may forget which seed is being used and retrieve a new one.
  • If you don't use a file editor that understands Unix newlines, your server.properties file might end up very screwy looking.

More info on server.properties: http://minecraft.gamepedia.com/Server.properties