Is it permissible for a female student to remove her hijab and wear whatever she wishes in a boarding school exclusively for girls in a foreign country, and is it permissible for her to join this school in the first place?

If there is no religious or worldly harm in enrolling in school, then there is no objection to it. The basic principle regarding things is permissibility unless there is evidence that necessitates prohibition, as Allah Almighty says: "And He has subjected to you whatever is in the heavens and whatever is on the earth - all from Him." And the Prophet, peace and blessings be upon him, said: "The lawful is what Allah has made lawful in His Book, and the unlawful is what Allah has made unlawful in His Book, and what He has remained silent about is what He has pardoned." It is permissible to place a partition (hijab) between women, but it is not permissible to wear tight clothing or to display what is not allowed, and this varies according to the circumstances of the women.