The modern practice in British English is to go up north and down south, taking over from the older system that JonathanBa309018 mentions. However the traditional use in Gaelic is to go up south and down north. Unfortunately the English is inevitably creeping in so it can be very difficult to know what someone means. I would guess it depends a lot on the age of the speaker and whether they are genuine first-language speakers. If anyone knows any specifics of what is used where and by whom, please post.
As for your comment that both are correct, that is true, whichever direction they mean! If you were given a fill-in-the-correct-word question then they definitely need to accept both or remove one. However I was given a type-what-you-hear question so only the one she actually said is correct.
If you struggle to tell the difference when listening, the trick is to ignore the vowels and listen to the s:
s as in soot = suas = 'up'
s as sugar = sios = 'down'
It is quite common in Gaelic to use the consonant to distinguish broad and slender vowels.
Edit: I have now had a click-the-tile question. I was offered shìos and shuas but they were both lenited, and hence wrong. So suas was the only valid option, whichever direction it means.
No it's not universal but very common. It used to be the case in the Uk that you go 'up' to town from the country and 'down' to the country (rural locations) from town. Hence the phrase "Freddy got sent down from University for flunking all his exams" (aka expelled). Because universities are nearlly always in towns, you also go 'up' to uni at the start of term and 'down' when you leave for the holidays. That said, it's getting a little old fashioned to use up and down in this way now. Which brings me to my question - is 'up' here north as suggested by AlanIlling or as in 'up to town' (ie geographic vs social).