But who would have known that Snape was wrong? Hmm. Makes you think about how that pompous exterior would have reacted, had he lived, to know he had given a classroom of students misinformation.
The truth is that Monkshood and Wolsfbane are not the same. However, a lot of people think that they are.
Monkshood is known as Aconitum napellus and has blue flowers, but Wolfsbane is actually Aconitum vulparia, and has yellowish flowers. Both are, however, quite poisonous. Wolfsbane is said to deter a werewolf (probably because they don't want to be poisoned). In ancient times it was used to poison the water supply of the enemy. The plants do serve some medicinal purposes, however, mostly topically.
So, teaching young children about the plants - well, maybe Snape was being helpful, or maybe he was being hopeful. He didn't seem to actually like children a whole lot, after all.