Any receptionist job, try car dealer ships, schools, daycare centers, many factories have a front desk girl and girls who do payrole, dog grooming places, many gas stations will let you sit other than moping at the end of your shift and doing stocking and garages, why do you have to be sitting? Doctors orders? Many women can do almost any job while pregnant. Also try all your local gyms and YMCA many times they have people who do payrole, calls for payments, updating memberships and online advertising.
Tha'ts a tough one, cuz most call centers want you to be graduated first, or at least have an equivalent high school diploma. I would def give it a shot, but you might find your self having a harder time finding a sitting job since your in high school still. Compared to getting a fast food job, waitressing, etc.
Call-center work is done sitting. Are there any near you?