Probably the reason they use cats is because they are smaller. I wouldn't have a problem using dogs either. It takes a lot of care to keep a disection in good condition. The materials need to keep, say a few 80-100lb german shepards, in good condition would cost a lot more money.
Is your problem here that "companion animals" are being used, or the disection itself?
My problem is with it being in high school ...on impressionble kids. I don't like the impression it gives them ...that 'companion animals' are just lab rats.