Is it obligatory to condemn a woman who wears tight trousers in front of other women, or to give her a note to that effect, and what if she is in middle school and her puberty is not known?

Enjoining against evil occurs after verifying its occurrence. There is no need to ask girls about their puberty; rather, relying on outward appearances and strong presumption is sufficient. So, if a girl is young and has not reached the age of desire (ishtihā'), then there is no enjoining against evil. However, if she has reached the age of desire and there is a presumption of being tempted by her, then enjoining against evil is permissible, even if she has not yet reached puberty. Advising and teaching those who are unaware of their involvement in evil is permissible, while observing the etiquette of advice.