Using "they" as a singular for the "abstract" case is pretty well-established. A sentence like this passes completely unnoticed by almost every native speaker:
"I don't know who took my tools, but they really caused a problem."
The less "abstract", the more apt it is to sound funny or new. But most of the controversy surrounding use of "singular they" is about non-abstract persons.
It's a complicated subject and you can expect usage to differ depending on who's talking or writing and what usage they grew up with. Historically, English teachers have often considered the male pronoun the "universal" one, so you would find "I would like to know a teacher and his students". I believe this usage to be outdated but it does exist. It's certainly the way I was taught but doesn't reflect how I use the language.
Sometimes you can say things in a different way to avoid the issue. For example, "I would like to know a class, including the teacher and students", or "I would like to know a teacher, as well as the students." Some of the connotations or emphasis can be different but this can sometimes work.