Why not: 학교에 가기 후에
shouldnt it be 가는 or is that not a thing?
가는 is currently going. 간 is "already went." Whenever using 후에 after a verb you'd always use ㄴ/은 to follow it.
는 = is
ㄹ = will
ㄴ = has
「학교에 간뒤」、the word「후(後)」can be replaced to「뒤(native korean)」
That's right!! Thank you for the contribution.
There's also 다음. And I read on a blog that 후 is pretty much formal compared to the other words, being used in very formal situations as well as in songs or novels. While 뒤 and 다음 are used in casual conversation, messages and such.