This issue is mostly for history and so that if somebody search informations about the problem, he can find them here.

Since pHP 5.6

a**b;

is equivalent to pow(a, b);

As a consequence, without any modification done to the code, it is now possible to use this operator in writing Formulas questions or during attempt for numerical and algebraic answers.

But this also require a very recent browser as the same change was only done in the browers javascript engines in 2017. So I suggest not to advertise this possibility too much to your students, and instruct them to use the a^b writing to enter answers to formulas questions.