I wouldn't do the cats in high school. I cried after seeing them because they reminded me of my own cat. I believe most cats are killed for the purpose of dissection, but slowly slowly slowly places that do dissections are starting to get animals from shelters and vets (yes your own pets could end up on the dissection table after euthanasia at the vet if you don't ask where they are going). I read somewhere that some places catch cats off the streets in Mexico and use them, but who knows. Imagine if you recognized the cat on the table and you were in high school. I volunteered at the animal shelter and my biggest fear was seeing one of the cats from the shelter in science class all cut up.

I did the frog though and I use the info as I've done necropsies on my own rodents when they die of unknown causes. But in all reality, most of what I learned about what the insides of a healthy mouse should look like and how to navigate their organs by looking at pictures, not by a classroom dissection.

I would never ever do vivisection (dissecting a live animal), unless you were actually doing a surgery. There's no point in keeping the animal alive while you cut it up just so you can kill it when you're done.