I also wrote an essay for another class (first-year university this time) and got full marks for it, but was still told that I didn't write enough. I'm not sure how that works...
I obviously cannot say whether this applies to you at all. But I went though this situation at high school. I was frequently told that I wrote essays that were clear, grammatically correct, and factually correct.
The only problem, I was told, was that I wasn't writing enough words.
I didn't understand why my teachers were getting hung up on the word count. Was I supposed to pad my essays out, I asked? "Well... no....," they would reply, unconvincingly. I never got a satisfying answer.
It was only years later, as I realised just how much better my writing could be, that I began to understand what my teachers were trying to get at.
At the time, I had seen the goal as merely proving to the teacher that I had absorbed the subject material. In truth, in asking you to write an essay, the teacher is testing whether you have the ability to successfully convey that information to someone who didn't already understand the material.
Merely laying the facts out in the most concise and grammatically correct fashion is only half the battle. I was submitting first drafts without getting even re-reading them myself let along getting someone else to proofread them for me. I wasn't putting effort into truly engaging the reader. I wasn't introducing the material in an accessible way. I wasn't telling a story.
I am not going to claim that my writing is brilliant now. But it is better than it used to be because I have a better idea of what I am trying to achieve. And unlike those days, my writing goes through at least a couple of drafts. If I don't have someone handy to proofread for me, I will at least put it down for 24 hours to separate myself a little from the writing before rereading. This will inevitably result in numerous corrections.
I would suggest finding an article from a writer that you admire on a similar topic to one you've written. Read that article a couple of times. Then go back and read yours. And ask yourself whether there is something that writer is doing that separates their work from yours.
Don't get too hung up on word counts, but you will probably find that your attempts to elevate your essay writing in that way will mean that instead of working to try and pad your essays out, you will be putting effort into trimming them before submitting.
TLDR You have to write a lot of words before you can arrive at truly great concise writing. Maybe your teacher sees writing that meets the standards for an A, but also sees its untapped potential.