the elementary school I worked in had it centrally located. I'm in a middle school now and have one near the gym -actually located in the cafeteria and i had one installed (brought over from my former elementary school, which closed and merged with another school in district) in the lower level of my bi-level school.
We don't have them at my schools, and I don't really want them. Kids are unlikely to need an AED and having them requires on-going maintenance and training of staff members. Also, I would worry about the liablility for the school if someone needed it and it was not used for some reason. (I am only at each of my schools part-time).
I would rather see that money used for programs to promote good nutrition and fitness.