If poor students have capable models but rich students have much better models that go the extra mile for a great mark and do everything in a single prompt, it would still be unfair.
For it to be fair, you would not only need good free models, but actual parity between free models and the highest subscription tier the big AI companies can offer. And I don't think that will happen in the short or mid term future.
When I was in AP classes in high school, you were required to have a TI-89 calculator. If you couldn't afford one, there were assistance programs.
You were not allowed to use a TI-92, which was the next step up. It had built-in solvers for many kinds of problems.
I'm not saying this isn't a concern, but addressing financially-based inequities in learning is not a new problem within certain bounds. There's established ways to deal with it. If we can get AI cheap enough that you can cover a year of education with $100 then we're in a good range.
For it to be fair, you would not only need good free models, but actual parity between free models and the highest subscription tier the big AI companies can offer. And I don't think that will happen in the short or mid term future.