ni-/-in-
‘school’ to be affected by vacation; hence, to be closed for vacation.
In the following example, the translation "vacation" in the first two stanzas, could be translated, "to be closed". To wit:
"Starting [now] school is to be closed/ To be closed for two months/". But this is too cumbersome for poetry, and the word
"vacation" carries all the meaning necessary.
From a song