There is quite a bit of shopping in downtown Glen Ellyn, from where Karen went missing. I got the impression she was dropped off to shop prior to working and work was in the downtown shopping area.
As for a student possibly having a crush on her, I don't know how many emotionally disturbed teenagers would have access to a car. It's about 14 miles to Geneva/Fox River from Glen Ellyn. Though, there the metra train runs between Glen Ellyn and Geneva and the Glen Ellyn train station is right where she would have been shopping, in downtown Glen Ellyn. It's probably unlikely she got on the train with someone.