I don't know weather it is of any importance but I was taught using arrive merely with at- so the sentence arrive in school sounds pretty odd to me and it ought to be arrive at school- I see the point in trying to translate the ni- but to my ears it still sounds strange.
Shuleni can be "at school" too. It's not always "in". It can be "to", "from" etc. as well. I agree with you! It's not really wrong but it sounds really weird to me to use "in" here.