Some rules in English - whether they are for spelling, grammar, or whatever else - are debated among linguists, vary between cultures, or simply depend on a teacher’s preference. For example, we at Insights favor the Oxford Comma, but we’ve noticed that excluding it is more popular than including it these days. Also, should you include ‘s after a singular verb that ends with an s? We say ‘yes’, but other sources will tell you ‘no’. So what should you do?
Here are a few issues that can be resolved in at least two different ways:
the Oxford Comma (the comma that comes after the second-to-last entry in a list)
British vs. American spelling (e.g. travelling vs. traveling; colour vs. color)
hyphenated words (eg. rat-infested sewers vs. rat infested sewers)
they vs. he/she for singular persons of unknown/ambiguous gender
possessive ‘s after singular nouns that end with an s (eg. James’s vs. James’)
words vs. digits for writing numbers (eg. twenty vs. 20)
There are others, and if you don’t have a solid stance on one of those particular issues, one could argue that you could decide how to follow the rule on a case-by-case basis. But for at least the six above, you should be consistent within each work.
For example, if you’re writing a story, you might choose to spell out a number. You should then continue spelling out numbers for the rest of that story. But then if you write a scientific report, you may choose to use digits to express numbers instead.
As a teacher, you should probably be consistent in what you teach students and how you grade their work. You may decide to explain to advanced learners that there are more than one options, but otherwise if you expect your students to write he/she instead of they, you should stand by that for all students for as long as that class lasts.
You can get picky with these rules, by the way. For instance, I personally use American spellings for most things (I write color and analyze instead of colour and analyse), but I like to use the double L after a vowel when adding a suffix (so I write traveller and cancelled instead of traveler and canceled). While I’m not consistent with the broad rule of British vs. American spellings, I am consistent with the more specific rules.
Again, you can pick what works for you. But you should pick one or the other.