Is it correct ?

句結構上沒大錯
假如是記述的話know應該亦是過去式才對...因為when前後的動詞(情形)大致都是同時發生吧...不會(因為)知道卻是現在而驚訝卻是過去發生-不合時空情理。
另外know的中文意思應該是「了解」或「知道」，但這句的情況其實所謂的「知道」英語思想是「發現/得知」的用詞，不是用know這認知的詞。

較為合理的寫法是：
Many people raised eyebrows when they found out he is a boy.

Many people raised eyebrows when they know he is a boy.
eyebrow==(noun), fringe of hair above the eye. 眉毛, 眼睫毛;
raised eyebrow==express surprise, doubt,etc.揚起眉毛(表示驚訝,懷疑等);
Many people raised eyebrows when they know he is a boy.
當他們知道他是男孩子的時候,感覺到非常驚訝。

Many people raised their eyebrows when they knew he is a boy.

Many people raised eyebrows when they knew he is a boy. = past tense
Many people raise eyebrows when they know he is a boy. = present tense

yes! it is correct!