why not "他们在学校的外面"？ That's how the rest of the lesson was taught..
both answers should be correct. colloquially, I would even say 他们在校外。 leaving the 的 in sounds a bit more childish
I'll use: 他们在学校外面
put 她们 as well
The 面 is not needed but its not incorrect. To use 外 as a preposition we don't need the 面。To say outside as a noun you do need to say 外面.
My question is exactly the same as elmanisero's question.
Sorry, I just realised that I asked this same question a year ago. Thank you Swansae.
why not "wai mian?"