Translation:They are outside the school.
"They are chillin' out maxin' relaxin' all cool and all shootin' some b-ball outside of the school." seems like more natural English to me.
They are outside of school
is also a correct translation and should be accepted. (reported)
outside 'of' is not correct! Outside is outside. No 'of' needed, despite a lot of people adding it in these days.
I find this awkward. Is this accepted phrasing in China? I would use "wai mian" here.
Yes, this is commonly said. If you want to use 外面 instead, then the sentence is 他們在外面的學校。/他们在外面的学校。
Yes, I am unsure why it is supposed to be clear that 'the' is necessary in the sentence...
Because the Chinese sentence is referring to location, and from what I understand, when referring to the location of "school" (i.e. the physical campus), I would always include "the." I would only not use "the" if I was referring to school in a more ambiguous sense of going to the place to learn.
You would and I wouldn't. Different uses of English in the various countries that speak it...
Could this also be "they are at a foreign school"? If not, is it because of the placement of "foreign"?