start school还是start the school

要结合语境

e.g.
I started school when I was six 我六岁起开始上学
I started the school when i was six 我六岁起就开始上XX学校了。注意，the school 必须要在上下文中有所说明，不然会造成歧义

都可以
the school是指定的学校