Teaching a class because advisor bought out and advisor input The situation is thus: my institution allows professors to buy out of teaching a class.  This semester, my advisor has done so and as a result they needed someone to fill in.  After discussion with my advisor, I applied for the position to gain teaching experience (having had none previously).

Thus far, teaching the class has gone smoothly but our department has a huge waitlist problem.  When I originally applied for the position, the class had 40 spots.  Later, without my input, it was increased to 60, and there were still over 50 people on the waitlist.

Prior to the class starting, my advisor attempted to get the department chair to increase the class size to 90 so that more waitlisted people could get in.  However, the chair responded by saying that the department felt that it was important for graduate students, particularly in their first attempt at teaching, to have a good experience, so they didn't want to increase the class size so drastically without my approval.  I responded telling them that I didn't feel comfortable taking on so many students when I hadn't taught before and the issue was dropped.

Now, in the second week of classes, my advisor is again pressuring me to admit any waitlisted students that would like to take the course, increasing the class size to 70+.  I've told him several times that I am not comfortable with this and thought the issue was closed.  However, one of the waitlisted students has gone directly to my advisor and now he's specifically telling me to admit this student.

As it stands, I currently have 62 students in a 60 person class and I understand that taking on one extra student is technically not a big deal.  However, I feel uncomfortable for several reasons: 


It feels like I'm rewarding bad behavior (e.g., the student going to my advisor and forcing entrance into the class)
If I admit this student for going to my advisor, I am concerned that every other student on the waitlist will go to my advisor and I'll get similar e-mails about all of them and suddenly my class size will be 70+ students


Technically, I don't have to admit this student, but that would be going directly against what my advisor is telling me to do.  

I guess the questions are: 


In the long term, how much control should my advisor have over my class?
In the short term, should I admit this waitlisted student?
In the long term, how much control should my advisor have over my class


None, for the duration of this instance. The advisor bought out. You're in charge. End of story. Now you might need some help in standing up to your advisor, and you need to bring in the chair of the department (who's already shown a willingness to help) and/or other senior faculty who manage curriculum activities. Don't do it confrontationally: merely say that you're really uncomfortable expanding the class size and don't feel like it's right to selectively admit students who have access to your advisor, but that you'd feel more comfortable with a faculty intermediary to help mediate. 


  In the short term, should I admit this waitlisted student?


No. you're right that this is both setting a bad precedent as well as being unfair. Again, as for how to proceed, see above. You haven't indicated exactly how the advisor is pressuring you, but I think it's fair to point out to them that this undermines your authority as the teacher, and you really need to maintain your independence because you're "only a student". Anyone with teaching experience should understand the importance of establishing authority and presence in the classroom.