Responses
I've only known it to be part of the indian culture. I did a quick search, and in ancient China, it also represents the opening of the third eye, as in wisdom, which in the context of this comic would also make sense. Apparently SH was questioning NK's love for Learn Shit. Also at the end of the chapter, the teacher reveals his true nature to NK. So we can see that gesture as SH wanting to put some sense into NK (Wake up my dear!!!). But I'm bias towards the wifie theorie uwu lol

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Ok so we're going to gloss over SH putting that red dot on NK's forehead ??????? I had to google it to make sure. it's part of the indian culture to signal marital status..Taken/Married...
"In addition to the bindi, in India, a vermilion mark in the parting of the hair just above the forehead is worn by married women as commitment to long-life and well-being of their husbands." Reference google.
People who are still saying SH doesn't have feeling for NK are in DENIAL !!!