My daughter will be 16 in less than a month.  When she was in middle school (age 12, 13, 14) she would come home every day after school and be home alone for a few hours.  It was just like with any other child, except she understands tht if she is not feeling well, she needs to let us know.  If she had a cold or something of that nature, I did not worry about it.  If she was running a fever (she would take her temp herself if she was feeling ill), nauseous, or having diarhhea she would not be left alone.  Even though she is almost 16 and going into 10th grade, I would not leave her alone for long if she was ill, and not at all if she was seriously ill.

If you are talking about being home alone under normal conditions (not sick) I would not be too worried.  Just be sure she understands the importance of contacting you right away if starts feeling bad and then you can make the decision about whether or not to get home right away.