I am a Muslim girl in college, and I end classes at 5:00 PM. I was thinking that I would stay on the school's library till 8:00 PM to study. Of course, by this time it will be dark outside and I will travel back home by myself with public transportation. It takes me about an hour to get back home. Is this permissible, or should I go home and study?
There are Hadith saying that a woman cannot make a journey alone "of a night", but this refers to the length of the journey rather than the time of day itself. This page has the sources and more explanation.
There are also Hadith which say that children should not go out at night because of Jinn:
Cover your dishes, close your containers, shut your doors, and keep your children indoors at nightfall, for the Jinn are out and about and stalking. - [Sahîh al-Bukhârî]
However, this only applies to children of course.
There doesn't seem to be any Hadith, and certainly nothing in the Qur'an, which forbids woman being out in the dark, unless it is making a long journey. So I think therefore it is a case of everything being Halal unless specifically made Haram...
You should of course consider your own safety travelling alone at night, but there doesn't seem to be anything haram about it.
This depends on the condition of your travel/route. If the route you travel has people around during your journey (i.e. no risk of being harassed or assaulted), then I believe it is not Haram to do so.
Of course it is always better and you're encouraged to study at your home after it gets dark. This is due to the fact that women are in much higher risk of assaults at the times of darkness.