Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
It is wrong for your daughter to imitate her schoolmates in not dressing modestly and in disobeying Allah.
She is about to become pubescent and she is a female who could be desired by men. It is possible for men to marry her, or take pleasure from her by looking at her, or touching her.
The Prophet (Sallallahu Alaihi wa Sallam) married Aisha when she was 9 years of age. Therefore, it is not permissible for you to let her wear shirts and trousers, as there are mischiefs in doing so.
Being dressed in that way shapes her body, and men could be tempted by her. It is also considered like imitating men, and the Prophet (Sallallahu Alaihi wa Sallam) cursed women who imitate men.
It is obligatory upon you to forbid her to dress in that way, and inform her about the good in wearing Hijab, as it is protection and chastity for women apart from its being an obedience to her Lord. This is our advice.
We also advise you to choose for her good female friends who will guide her to the obedience of her Lord and help in doing so, and to keep her away from the bad female friends who do not dress modestly, as they could affect her in her conduct and in her religion.
Allah knows best.