I do everyone else’s jobs all day on top of my own. Possible to stop this or do I need to leave?

I work for a private school and was recently “promoted” to a new position focusing on marketing (I haven’t had any “official” conversation or pay increase but they’re having me take on these new job duties and gave me a new title). I thought this would actually improve things, but nothing has changed.

Since I’ve been with them, I’ve felt that most of my coworkers are incompetent/do not have the skills necessary to succeed in leadership roles. Many are extremely unfamiliar with basic computer functions and I’ve had to do many things for them over the past year and a half. All day I’m helping others write emails, make documents, do billing, and filling in for other random things. Admin and directors who should know how to do basic office things do not know how to do these things and never seem to retain what I show them. Things constantly get done wrong and I have to redo. I’m at the end of my rope with this and feel burnt out. How do I approach this conversation with my boss/the school owners? I’d feel bad saying that my coworkers are incompetent but.. it’s just true. I want to have the time to do my job WELL, and not constantly be picking up the slack for everyone else and then fall behind on my actual duties.