When I was in grade school, we were taught that live birth was one of the traits of mammals in general, but that the monotremes were just really weird mammals.
Me, too. I was in elementary school in the 60s, at which time cladistics had not taken hold. The emphasis was on physical descriptions.
Mammals = hair + milk + live birth + warm blooded.
No need to let those messy monotremes ruin such a tidy definition-- just include them as an exception. Especially when you’re teaching kids.
Actually, as an operational definition for living forms, either hair (with the structure found in mammals; other organisms have hairlike features with different structure) or the production of milk from mammary glands by themselves are sufficient to define a mammal. Warm blood and live birth (except for monotremes), while found in all mammals, are also found in other vertebrates.
Likewise, as an operational definition, feathers are sufficient to define modern birds, since no other organism has them. However, the discovery that many dinosaurs (that is, non-avian dinosaurs) also had feathers makes it necessary to use a different defining feature for the clade Aves.
I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus
Given his vagueness about other aspects of what he was taught, I think the “bad student” hypothesis is at least as plausible as the “bad teacher” one. 
Thanks for the clarification.
So nobody cares about “four chambered heart” anymore? That was always one of the characteristics I was taught as a child. (And that, duh, monotremes and marsupials are mammals.)
Aren’t those suckers poisonous?
The males are - they have hollow spurs and poison glands in their hind legs. And a nasty disposition.
For the layperson, the main common defining characters of mammals are mammary glands (but not nipples, as mentioned above) and hair during some point in development. Not live birth. As stated, many organisms have live birth, but only mammals have hair and mammary glands.
To the taxonomist, there are other defining characters (ear bones, heart structure etc…) that are useful.
This is the problem with bad science textbooks being taught by teachers with no science background. They simply teach the book as gospel.
The males are. They have a spur on the each of the back legs for injecting venom. In humans it mostly just causes some swelling and pain, lots and lots of pain. It won’t kill you, although you might wish it had over the next few days (weeks or months if you’re really unlucky) as the stuff works it’s way out. Oh, and according to a report from 1992 not even morphine was able to block out the pain.
However the glands are only active during the breeding season. So would a neutered platypus produce venom?
I’ve not been close enough to a platypus to pet it, but I have been close to echidnas, both in the wild and in captivity – and I have patted one. Those spines look sharp, but in fact they’re pretty blunt, though pretty hard, so it isn’t like patting an animal with normal fur.
Pedantic nitpick: animals that can kill you with a toxin by getting that toxin into you via injecting it through your skin are venomous, not poisonous. Poisonous is a plant or animal with a chemical that sickens or kills you when you ingest it or touch it.
So a male platypus is venomous, not poisonous.
Carry on.
What if you eat their venom gland?
Then you become eligible for a Darwin Award.
Don’t even try. The girl just looks at you funny.
While all mammals have a four-chambered heart, again it is not a character unique to them. Birds also have a four-chambered heart, as do crocodilians also (although crocodilians have a special shunt between the arterial and venous systems).
This is not really correct. In common usage, it is perfectly correct to refer to animals that inject toxins as poisonous.
From Merriam-Webster:
The distinction between venomous and poisonous you mention is made in some technical usages; it is not necessary in informal usage. It is perfectly correct to call a rattlesnake or a platypus poisonous.
I (as a kid in the 70s) remember learning that the mammals certainly included marsupials, and that platypuses were weird precisely because they’re mammals that lay eggs, and that all vertebrates are either mammals, birds, reptiles, amphibians, or fish.
They Might Be Giants agree.
It’s a large four-chambered heart, maintaining the very high metabolism rate they have.
(On preview: Hi, Thudlow!)
Expanding on the four-chambered heart point: the one dinosaur fossil which may have preserved thoracic soft parts (whether they are or not is hotly debated) is a Heterodontosaurus, a human-sized ornithopod which is exhibited about 20 miles west of where I sit typing, at the North Carolina Museum of Science. There is a concretion in the thoracic cavity that may or may not be the heart – and it appears to have four-chambered structure. (For anyone wondering why this is important, having four chambers allows for separate pulmonary and systemic circulation: deoxygenated blood is brought into the right atrium, pumped to the lungs by the right ventricle to be reoxygenated, then returned to the left atrium, then pumped out to the body by the left ventricle. Fish, amphibians, turtles, and lepidosaurs (lizards, snakes, tuarara) mix oxygenated and deoxygenated blood in a single ventricle. Birds, mammals, and crocodilians on land maintain the double circulation, but crocodilians have adapted to reuse the (partially) deoxygenated blood while submerged and not actively breathing.)
Ya know, this sounds awfully familiar. I may have been taught that myself back in Texas, say slightly earlier than that time period.