Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
It is Haram for a Muslim woman to wear trousers (without a covering cloak or dress) while she is outside her home, i.e. at work, street, school, etc… Wearing trousers under one's cloak is better and safer. But, wearing it without a cloak or dress is, in fact, a form of imitating men; such an imitation is Haram since the trousers expose the shape of a woman's body, the thing which leads to temptation of men.
On the other hand, a Muslim woman can wear what she likes provided that her dress should not (1) describe her body, (2) be transparent, (3) perfumed or incensed, (4) or be like the dress of men, disbelieving women or prostitutes. Also Muslim woman should not wear a dress for the sake of adornment and fame. A Muslim woman has to comply with her Islamic Hijab, whether she is in a Muslim country or a non-Muslim country. Yet, it is more Haram to expose her ornaments in front of non-Muslims. A faithful Muslim has to pride himself on being a Muslim, not to be defeated to put his/her ideas into practice while in non-Muslim community. Rather, he/she has to do his/her best to produce an effect on non-Muslims, not the opposite.
Allah knows best.