Both of them are correct. As a native Chinese speaker, I really can't tell what is the difference between them. I always use both of them. Put "了“after a verb, it always indicates the action happened in the past. But when it comes to "我去学校（school )了“ and ”我去了学校'',maybe they are different. Why I use MAYBE, because it depends the context. If someone ask you" 昨天你去哪儿了”，and you answer"我去学校了”“我去了学校”，both of them are right,and the meanings are the same. If you are going to school and say to your mum" 我去学校了", here "了“just like "呢”“啦”， doesn't have meaning. You can't say to your mum" 我去了学校”。

i hope this can help you!:_) 希望能够帮到你。

 Both correct.

for example, if your friend went to a trip, you can ask "你都去了哪？"

If you can not find your friend after you went to buy a coffee when you guys are hanging out, you can say 你去哪了？ when you find he/she.

儿 means nothing in this two sentences. You can delete it. 

BTW, it is weird to say 再见! at the end of a post. Just say thanks would be fine. 

“你去哪儿？”it's a future tense,and this means “Where will you go？”But “你去哪儿了”，it's a past tense. You should use this form. And give you a hint. In China “了”always represents past tense. Haha

                          Hope I can help you(^_^)

                                                            Grace