Other words such as altogether = all together are examples as well.
Bad example, since "altogether" is a legitimate word and has a completely different meaning than "all together," they mean "completely" and "in unison" respectively. Ex. they are altogether different words, we all together can agree with that
And popular usage is how words come to be an accepted part of the English language, otherwise we'd still be speaking like the pilgims in a Chaucer's Canterbury Tales and saying "thee" and "thou" and such. Heck, maybe the OP should write in Middle English just to see the teacher's reaction.
1: The ministre and norice unto vices
2: Which that men clepe in englissh ydelnesse,
3: That porter of the gate is of delices,
4: To eschue, and by hire contrarie hire oppresse,
5: That is to seyn, by leveful bisynesse,
6: Wel oghten we to doon al oure entente,
7: Lest that the feend thurgh ydelnesse us hente.
8: For he that with his thousand cordes slye
9: Continuelly us waiteth to biclappe,
10: Whan he may man in ydelnesse espye,
11: He kan so lightly cache hym in his trappe,
12: Til that a man be hent right by the lappe,
13: He nys nat war the feend hath hym in honde.
14: Wel oghte us werche, and ydelnesse withstonde.
15: And though men dradden nevere for to dye,
16: Yet seen men wel by resoun, doutelees,
17: That ydelnesse is roten slogardye,
18: Of which ther nevere comth no good n' encrees,
19: And syn that slouthe hire holdeth in a lees
20: Oonly to slepe, and for to ete and drynke,
21: And to devouren al that othere swynke,
22: And for to putte us fro swich ydelnesse,
23: That cause is of so greet confusioun,
24: I have heer doon my feithful bisynesse
25: After the legende, in translacioun
26: Right of thy glorious lif and passioun,
27: Thou with thy gerland wroght with rose and lilie, --
28: Thee meene I, mayde and martyr, seint cecile.