Context: the student council president Amakusa Shino 天草シノ, who has a complex about her breast size, sees Hagimura Suzu 萩村スズ, who has a complex about her stature, drinking milk.
Hagimura wa gyuunyuu ga suki nanda na 萩村は牛乳が好きなんだな Hagimura, [you] like milk, don't you?
gyuunyuu wa {seichou wo unagasu} mono dakara ii koto da 牛乳は成長を促すものだから良いことだ Milk is a thing [that] {stimulates growth}, so it's a good thing.
A jab at the fact Hagimura is a shorty.
*Hagimura stares at the student council president's chest.*
kaichou wa gyuunyuu kirai desu ka 会長は牛乳嫌いですか Do [you] hate milk, president?
kaichou 会長 [Student] council president. (used as a title. translated as "you" here.)
yokumo mesen shita ni mukete kureta na よくも目線下に向けてくれたな [You have some nerve] to point [your] line of sight downwards [and say that]. [You have some nerve] to [look] downwards [and say that].
shita ni mukeru 下に向ける To point below. To point down.
~te-kureta ～てくれた To be given or done something by someone. (in this case, it refers to the act of Hagimura of looking downwards. How dare you "give" this to me? How dare you "do" this to me?)
