Yeah, 'girl next door' is too close to a cliche. How about: "She looks like that substitute teacher you liked so much."
What if there wasn't one?
Did you still create an image in your mind's eye?
I had to stop and think. I could only think of two substitute teachers that I remembered. One was a guy. Does your character look like a guy? I don't know. I mean, I wouldn't describe him as "that substitute teacher you liked so much." I didn't not like him. I guess I liked him fine, but I wouldn't describe him as someone I liked "so much." That kind of implies I looked forward to seeing the substitute teacher or something. I met him again some time after graduation when I made copies of his resume for him. (I work in printing.) He seemed to be a nice guy out of the classroom too and I hope he got a good teaching position somewhere. The second was a woman. If you meant "liked so much" in a sarcastic way, then maybe it works. I don't remember what she looks like. I only remember that she was pretty much a horrible person that should probably never be around kids. I cannot believe the school system later hired her as a full-time teacher and apparently not the guy. On the plus side, I guess that meant she didn't come to us for resume copies and, thus, I never had the misfortune of seeing her again.
And now I kind of vaguely remember another substitute teacher. She was okay. I guess since I only vaguely remember her, it'd be difficult to describe her as one I "liked so much."
So, I don't know. Such a description would tend to throw me out of the story as I tried to think of someone your description wants me to think of.
Bear in mind too that some references may force the reader to dig deep into their memories, depending on age. A college-aged reader is probably going to have an easier time remembering a substitute teacher they had than a retiree-aged reader.
I'm all for experimentation in writing. I mean, I've had characters punch the reader, for example. But, I'm not sure how this one is going to work. If you're too non-specific, you run falling into clichés as you've already observed. If you're too specific, you risk throwing the reader out of the story trying to come up with something--more so if they have no frame of reference, such as if you describe a character as looking like "that substitute teacher you liked so much" when the reader was home-schooled.