I took some German years ago. Using this as a refresher. A lot of people are having trouble with gender roles of articles and words.
You can't make assumptions like "die" being used for "feminine" words. The same goes for "der" with "masculine" words and "das" with "neutral" words. You can say that MOST LIKELY "die" is used for a "feminine" word. Same goes for the other two articles and their respective "genders". I'll give an example. Das Madchen (Sorry, I don't know how to get the umlaut over the A in here) translates to "the girl". Girl is most definitely not neutral. It's clearly feminine, and yet it has "das", instead of "die". That's what I mean when I said you can't make assumptions, and that you should tell yourself that articles of a certain "gender" MOST LIKELY go with things of that "gender". For those looking for a more in depth explanation of this question, the answer is "die Lehrer", because it is the plural form of "der Lehrer". There are multiple teachers. At least one of them is male. Bunches of people always revert to the male form, not the female form. I'll give you an example of what I mean. A male teacher translates to "der Lehrer", while a female teacher is "die Lehrerin". Now say there's a group of 10,000 teachers. 9,999 can be female (Female teacher - singular form: die Lehrerin. Plural form: die Lehrerin) and only 1 male (Male teacher - singular form: der Lehrer. Plural form: die Lehrer). The fact that there are multiple teachers makes the noun plural. So it will now be either "die Lehrer" or "die Lehrerin". What decides which "gender" is used is the fact that there's a male teacher in that group. It doesn't matter if it's 1 out of 10,000 or 10,000 out of 10,000. If there is a male member of the group, it causes the group to take the plural form of "der Lehrer", which is "die Lehrer". I'm pretty sure I am correct about all of this, but if someone knows I'm wrong, please let me know. I'd hate to think I'm right about all of this and continue thinking the wrong way.
There are definitely certain rules to follow with regards to genders, it's not all about common sense. For example, the diminutives -chen and -lein are always neuter.
That is why you say 'das Mädchen', even though you're referring to a girl, and 'das Fräulein', even though you're referring to a woman.
Bonus: most but not all nouns that end in -e are feminine (e.g. die Farbe, die Reise, but das Auge).
I'm German and it is not right: When there is a mix of gender within teachers we tend to use both plurals: "die Lehrerinnen und Lehrer" to not disadvantage the female ones. Also there's a mistake: "So it will now be either "die Lehrer" or "die LehrerinNEN"." Otherwise you compare "the teachers" with "the female teacher".
der Lehrer (masculine, singular), die Lehrer (masculine, plural)
die Lehrerin (feminine, singular), die Lehrerinnen (feminine, plural)
The masculine form can represent mixed groups as well.
You just need to learn the der/die/das (and the plural form, but there are some regularities for plurals) with every word :-( So instead of writing down "Stadt - town", write "e Stadt - town" etc.
Sometimes you can guess, like for professions (and other words that mean persons) the female form usually has -in at the end. Words that end with -er are usually masculine, those with -chen or -lein are usually neutral.