What gives us the right?

Today I had to participate in my third vivisection. If I didn't help, my physiology teacher would fail me for the lab. There was no option not to, no matter how much you disagree morally with the experiment. The first vivisection was for a frog's femoral nerve. He was a big dark brown toad, and when he was brought to our lab table, the top of his head had been cut off with scissors. It was a grotesque sight, but with no eyes, it was a little more tolerable than the next two. All during the lab his legs twitched and blood seeped from his severed face into his quivering mouth. I almost fainted. The second vivisection was for a frog's gastrocnemius muscle. This frog had been "pithed" the traditional way, where a needle was inserted through the base of its skull and then twisted around, scrambling its brain, "ending its pain." It was then pithed a second time, the needle inserted down the spinal canal to destory the spinal cord. As my lab partners laughed at how the frog's skin came off "like a pair of pants" and squealed at how slippery the living leg muscle was, I just sat silently. I could feel the glassy eyes of the frog staring at me, and when I dared to look, I could have sworn I saw it crying. Then the professor took the bone cutters and clamped down on the femur, ripping the bone from the frog so that we could "more easily handle the muscle." The frog's toe never stopped twitching.
Today was the third, and thankfully last vivisection of the class. Again the frog was double-pithed the traditional way. This frog was smaller than the others, and greener, and cuter. Its legs twitched more often and forcefully than the previous two. The professor said he might have messed up the second pithing a little, but that we shouldn't worry, it still couldn't feel. My group carefully lifted the skin above the sternum and snipped it away. Then carefully they pulled up the sternum, revealing a tiny beating heart. Coldly, we went through the experiment, measuring the changes in the heart rate when different chemicals were applied. This was the first vivisection I actually participated in and was starting to think I had become desensitized to the inhumane procedure. Then as I got more "frog ringers" ready to drench the heart, my lab partner suctioned some liquid from around the heart. Suddenly both the frogs legs kicked out desperately and then crossed, twitching horribly. I cried out. It was horrible. Why is such an experiment legal?
Okay, I know, I know, supposedly the pithing ends all pain, which I don't know if I completely believe, but even if that is true, I am pretty sure that the actual pithing procedure does involve pain. A needle jabbed into the back of the head is bound to be excruciating, and if this procedure was done on humans, it would not be tolerated. This is the kind of "experimenting" the nazi doctors did. Why should it matter whether it is a frog or a human being dissected alive? How can students be forced to do such experiments and be traumatized for life? And with the modern technology we have, why must so many innocent animal lives be sacrificed for "science" whenever computer programs exist to teach students the same information without any life being taken?
Now, I am no huge animal rights activist or anything. I like some meat, i'll admit i've worn fur. It's only whenever something so horrible and unnecessary is shown to me do I really feel the need to step in. I hope someday the pithing will stop. If it doesn't, just watch, someday aliens will come and abduct us and perform the same experiments on us, for compared to them we might be as unintelligent and insignificant as a frog.