Which gender to use when speaking about people?

I’m not really sure how to phrase this, but my German teacher says that when we talk about people in plural and singular, we have to say the version of the male and female equivalent - i.e. Students would be: Schüler und Schülerinnen, w/ a journalist would be: mit einer Journalistin oder einem Journalisten, etc.

Is this a grammatical rule in German? Cause i remember a while ago a native German speaker told me as long as there’s even 100 girls and 1 boy, you say the male version and not both, only when you’re talking about specifically women do you say the female version, otherwise if its mixed then say the male version.

I just wanna check if they’d mark this as a mistake for when I’m doing my German exam