Maybe you're assuming too much when you take it as a fact that you can do a better story with more depth & more substance & themes & oh by the way more characters with fewer words. Really?? You have less than 2300 words there! Did the teacher really say you couldn't make the story longer?
Your teacher must be teaching something if he assigned such a short a max length. I've no idea what though. I can understand what a teacher is up to when they assign a minimum length. Maybe we should see the original assignment verbatim before analyzing the teacher's motives too far.
Your teacher also seem to be doing something arbitrary, anyway, when he says you haven't enough characters. Unless perhaps they had a minimum level of character building efforts in mind?? After all, some great stories have been told with just ONE character.
Maybe the teacher isn't really trying to help you write a good story as much as they are forcing you to use your noodle, somehow. I think you should talk to the teacher about what they expected from a less than 3000 word story with 3+ characters & a lot of depth & substance.
Personally, I think you've done a fine job of painting who these two people are & where they may have come from. Especially when done in only 2291 words.