"Hasta olacaðým için yarýn okula gidemeyeceðim." and "Hasta olduðum için yarýn okula gidemeyeceðim." are both correct, but there is a difference.

According to first sentence, you are guessing that you will be sick tomorrow, maybe there are some symptoms. But you are not sick yet. However, second sentence shows that you are already sick, and since you will continue to be sick tomorrow you won´t be able to go to school.

So, if I´m planning to take a, erm, "sick" day at work, I could say "Hasta olacaðým için yarýn iþe gidemeyeceðim"