The true value of Mathematica lies in its vast, high quality, integrated data sets and domain specific algorithms. As far as a teaching tool for math, it doesn't make sense to lock into a proprietary tool.
I'd say jupyter notebooks surpass Mathematica UX wise.
I maintain Mathics [1] which has it's own web interface e.g. [2]. I don't think the Mathics interface is as nice as MMA but the documentation bar on the side is really nice.
