Some would argue that humans have little, if any duty to non-human animals because humans are more important, and if keeping animals in zoos serves any educational or entertainment purposes, it serves a purpose to the humans, even if that purpose is not beneficial for the animals.
All of that comes before the actual direct education that can take place through signs, talks and the like that can directly communicate information about the animals they are seeing and their place in the world.
If zoos are teaching children anything, it's that imprisoning animals for our own entertainment is acceptable. Without them, the world would be, and would increasingly become, a much poorer place. They list the following weaknesses: a zoo may be unable to keep a large enough number of individuals to provide a sufficiently varied gene pool for the species to breed without problems where animals are rare and hard to breed in captivity, removing specimens from the wild to zoos may result in the population falling returning animals to the wild is difficult the benefits to the overall species population do not compensate the individual animals for the negative effects of living in a zoo.
There are counter-arguments, of course. They can be spared bullying or social ostracism or even infanticide by others of their kind, or a lack of a suitable home or environment in which to live.
A species protected in captivity provides a reservoir population against a population crash or extinction in the wild. I could never be sure if the cat coming back was a thumbs-up for the family, or a silent protest against the lack of amenities in town.
Removing individuals from the wild will further endanger the wild population because the remaining individuals will be less genetically diverse and will have more difficulty finding mates. Being able to study animals in zoos where there is less risk and less variables means real changes can be effected on wild populations with far fewer problems.
If we are to save many wild species and restore and repair ecosystems we need to know about how key species live, act and react. Sure there is always scope for improvement, but these benefits are critical to many species and potentially at least, the world as a whole, and the animals so well kept and content, that I think there can be few serious objections to the concept of good zoos what they can do.
Animals in captivity suffer from stress, boredom, and confinement.
Yep, captivity is certainly cruel to an extent.