Internationale schools...again.Help. (Athens)

Hi,

we plan to move to Athens and will go for a look-and-see-trip from tomorrow on.(I look forward to some nice warm days-it´s Winter in Sydney)
We´ve already organised some school visits in the northern suburbs from Athens-St.Catherines,Campion,ISA,ACS.
Just from the websites all the schools look great.
But I´m getting a bit nervous now since I´ve read about the entry regulations.
How hard is it to get entry into the above mentioned schools?What happend if we can´t get our kids in any of these schools?
Our kids are 8 and 12 years old.They attend an australian school since 3 years and are therefor fluent in english.(Mine is still improving ;-),greek might suit me better? )
Any advise or shared experience would be greatly appreciated.
Thx!!!