I have to agree. Of the answers posited so far, “ruler,” to me, by far fits the best. Plus the “too” in the rhyme also seems to suggest to me that the riddle is fishing for a double meaning word.
I don’t like it. A ruler can be a man OR a woman, but I can’t think of one that was “both man and woman too.”
Well, the other thing I don’t like about my initial though of clownfish, besides nitpicking about the technical definition of school and that they, also, don’t really fit being male and female at the same time, is that the words use are Man and Woman, which to me suggests the conventional definition referring to a human being. I would never refer to a fish as “man” and “woman.” I understand your objection, with “or” being the conjunction that better fits with “ruler,” but it still seems like the likeliest answer of the bunch suggested so far.
Ok, from what I can find, the riddle is fully:
From looking at other rebuses at around this time, it appears that a rebus (of not the pictogram type) is a type of puzzle in which the parts of the clue lead to the answer, see for example here. (Click on page 190 to see an example rebus and an answer.)
They don’t seem to be straightforward riddles, but based on wordplay or initials of the various answers to clues in each line of the rebus, but I’m not 100% sure and this particular rebus is much shorter than the typical ones I’ve seen examples of.
For all I know, though, “rebus” at the time might just generally mean any kind of wordplay/pun puzzle, to which double meanings like “ruler” would fit.
Thanks for the link, Pulykamell - I checked it to make sure that the capitalization of AM was a stylistic/typesetting convention and not a clue. It appears all the entries capitalize the first word of the puzzle, or the first two words if the first word is one letter. So there is no significance, most likely, to the capitalization.
Did neither of you read post #19?
Um… I guess not. 
I’m also wondering if something like the answer to the first part could be an anagram of the second part, as it seems some rebus answers I’ve found seem to be a series of anagrams, but I can’t find the original rebus puzzle to check. That said, I’ve tried anagramming Tiresias, Teiresias, Hermaphoditus, and hermaphrodite to no avail.
ETA: I should add that it’s not specifically one letter per line (as in you mentioned in your post). There are some rebus answer, like the “murray” one I linked to, in which it’s the first half of the word “murder” + “ray” to get the answer “Murray.” So it’s almost like a cryptic crossword clue. Plus there’s a set of rebus answers that I mention in this post that were three words that were all anagrams of each other, but I can’t find the original puzzle now.
ETA2: Here’s the one with the anagram answer to some rebus without the original rebus included. So the term seems to be a broad term for various types of word/letter-play.
Actually, I’m not so sure about that. I wish I could see the original rebus that was answering. Most of the poem-style rebuses I can find do either seem to be of the initials of answers to part of the rebus forming a word at the end, or of the syllabic type.
Here’s the original rebusto which Snail/Slain/Nails is the answer. (scroll up a few lines)
Good find. So it looks like that rebus makes it clear that it’s an anagram, literally telling you to transpose the letters of the answers to each section of the puzzle. I can’t seem to find any rebus of as short a length as the OP’s–they all seem to be fairly long affairs.
I would think the “go to school as good boys do” would imply that it’s something that a good/reverent child would take to school. I’m trying to think of something like a “hymnal” that would satisfy both the male and female aspect.
Considering the information about the rebus, I was thinking something along the lines of Angels/Angles or something (good boys, both man and woman, something school related), but I admittedly can’t get that to work exactly.
There’s one thing I’ve thought about. The answers to the puzzles are often themselves short verses. Maybe we’re seeing the answer, and it’s the puzzle that’s missing?
LAD, LADY, DAILY adds a letter…
Oh, I like that-- moreso if it was phrased “I contain both man and woman” and then the answer could simply be “daily.” (assuming that works as an answer for good boys going to school. I mean, technically, there’s days off, but I think it works in the spirit.)
The answer is at the bottom of this page.
Okee dokee.
Nice find. I tried looking for that issue, but couldn’t find it (I was looking on Google books.) So, the answer is just that mom, dad, and son are all three people named “I”? Well, that’s disappointing.
(ETA: If you found the answer yourself, you might want to contact Will Shortz with it. I’m not going to steal your thunder. 
)
Well found, and what a stupid puzzle. It’s like “The third word in the English Language.” :mad:
Well, the other thing is that in order to conjugate it as “I am,” “I” must be used as a first person pronoun. Otherwise, if it’s being used as a name, it should be “I is both man and woman” (or, better yet, “I is both a man and a woman.”) Now, that “answer” has what appears to be a name attached to it or something. I see “U Stow” at the bottom of the answer. Now, from what I can of the rebuses, people submit the answers and the editor picks the best poem answer and publishes it. I assume they check with the rebus writer’s intended answer, but this really is a weak one and I wonder if they just published whatever seemed to fit.