Why the default value of static class is zero?

Assuming you mean "variable" not "class" as C doesn;t have any concept of classes and all your questions a tagged as (and appear to related to) C rather than C++ or C#!

Compiler dependant, mostly.
In C, static has two uses, depending on where the variable is declared.
1) Outside a function. In this case, all is does is say "this variable is only accessible within this file: nothing outside the file can see it". Which means that you can have two variables with the same name in different files, and they won't interfere with each other.
2) Inside a function. In this case, it says that no matter how many times the function is called, it always gets the same instance of the variable: which means that if your function changes the value, the next time it is called it will have the new value. You need to be very careful here - this is very dangerous in multi threaded applications!

The initial value depends on the compiler, just as it does for all other variables.