Okul çıkışında bizleri alıp Ortaköy´deki çay Bahçesine götürdü. 

Bizleri yukardaki cümle ne anlamı var ve neden bu şekildeki kullanmıştı? 

Şimdiden trşekkür ederim. 

"...yukarıdaki cümle ne anlama geliyor ve neden bu şekilde kullanılmış?"

He could´ve said "bizi", too; bizleri is kind of a colloquial way to say the same thing. In every language there´s the written language and the spoken language. This use belongs to the second and is quite popular. Another famous example of this kind is:

"Teyzemlere gittik." (We went to our Aunt´s house. Meaning our aunt and her family, or her friends; this term is quite vague.)

"Annemler geldi!"

It could mean "mother and father", "mother and son"; anything, really. It depends on the context; if the person whom you´re speaking to knows that your mother left with, let´s say, your brother, he´ll know that it means "mother and brother". It´s just a figure of speech to put it short.

Hope it was helpful,

-Jansay

Actually Bizler = Biz, they are same. Kind of double plural as biz is already plural plus an additional plural ending [lar]  

These double forms biz --> bizler and siz ---> sizler  are seen in emphatic , formal , ceremonial way of speeches.