Spawn

you can use world edit and put your spawn on the normal spawn but if you really want it on that place you should search for a good plugin where you can do this
I hope i were able to help you
Greets
Harry!

You want to put a lobby, for this you need two pluggins:
1. Lobby: Use the command /setlobby in 145 4 767 URL: http://dev.bukkit.org/bukkit-plugins/lobby/
2. Joincommands: This plugin is not in Aternos but I do the request

You can use

/setworldspawn

to redefine the spawn point.

but he wants that every player spawns exactly at these coordinates

He can use a plugin and set a warp possibly

Quote from ScrackHD

but he wants that every player spawns exactly at these coordinates

Every player will spawn around that coordinate if you use the command I recommended.

yeah but only around...

You can set a command block in the area where they spawn and make it spawn over where you want it. And then set a spawn point command block with a pressure plate so there spawn point sets there.

/setworldspawn (sets the world spawn at coordinates)
/gamerule spawnRadius 0 (sets the spawn radius to 1 so people spawn at exact coordinates)

I hope I helped you.