AT shcool,it is essential that every child--equally regardle

C：be treated 是正确答案.
在 it is+ necessary/important/essential 等形容词+that 从句的情况下,that 从句中的谓语部分要求用虚拟语气,即should+原形动词,should 常可省略.具体到这个that 从句,由于every child和treat之间是被动关系,需要用被动语态,也就是should be treated.由于省略了should,就只能选C “be treated”.