Should she continue her studies in a mixed school where there are no Muslims?
We praise Allah, may He be exalted, for having enabled you to adhere to hijaab and we ask Him to increase you in guidance and steadfastness in following the truth.
Mixing between men and women in schools, workplaces and so on involves many risks and evils, and its negative and harmful consequences are clear and cannot be denied, even in Western countries themselves.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
When men mix with women, it is like mixing fire and wood. End quote from al-Istiqaamah, 1/361
For more information, see the answer to question no. 1200
The danger is worse in the school where you are studying because there are no Muslims there.
Based on that, it is not permissible for you to carry on studying in this school, and if you have a permissible means of learning that does not involve mixing, such as online learning and so on, this is what you have to do. If that is not easily available, then you should move to another school where there are Muslims; this is less bad than where you are now.
You have to avoid mixing with men/boys and speaking to them as much as possible, and let your friends only be Muslim girls. For more information please see the answers to questions no. 113431, 72448 and 45883.
With regard to your saying “how should I interact with my male friends and female friends?”
With regard to your male classmates, it is not permissible for you take them as friends.
With regard to your female friends, you have stated that they are not Muslim. The Prophet (blessings and peace of Allah be upon him) urged us to make good choices of friends and choose those who are religiously committed and of good character, because a person will be influenced by his friend and will imitate him. And the Messenger of Allah (blessings and peace of Allah be upon him) said: “A man will follow the religion of his close friend, so let each of you look to who he takes a close friend.” Narrated by Ahmad. 7968; classed as saheeh by al-Albaani in Silsilat al-Ahaadeeth al-Saheehah, 927
It says in Tuhfat al-Ahwadhi: i.e., a person will follow the habits, way and behaviour of his friend, so let him think long and hard about who he takes as a friend; if someone's religious commitment and character are good, then take him as a friend, otherwise avoid him, because the behaviour of your friends will influence you and the company you keep has an impact, be it positive or negative
With regard to your saying that you do not want the people around you to have a bad impression of Islam and Muslims, and think that Muslims are antisocial and did not like to meet their friends, especially when they have been accepting of these changes you have made your life:
Islam does not tell Muslim to keep away from people and not have any friends; rather it enjoins us to keep away from evil and to avoid places of temptation, and to keep away from people who may cause you to disobey Allah.
What is required of the Muslim is to adhere to the rulings of sharee‘ah and give a good impression of Islam to non-Muslims, by showing them that Islam is a religion of chastity, modesty and good morals, and that it does not permit its followers to follow whims and desires and temptations. Islam forbids immorality and evil, which includes free mixing between men and women, as mentioned above.
We ask Allah to guide you to that which is good for you in this world and in the Hereafter.
And Allah knows best.