First off both dramas have a duo that stick up for each other
Second; both dramas take place in a high school area
Third; many similarities in characters as in parents' absence/issues, too much pressure on the high school student, etc.
Fourth; both are about revenge it's just weak hero class 1 is a bit more thriller kind rather than hs return of a gangster where its mostly just fist fighting and knives.

Flex X cop is very similar to tomorrow! it can literally be ranked as number 2 closest besides taxi driver. The male lead in this drama reminds me a lot of the funny, immature one in tomorrow that would bother the female lead. Every episode of Flex had a new plot where the detectives would solve murder cases rather than suicide cases!! I definitely recommend watching Flex X cop, it's so good I can't bring myself to finish it!!