Isotriviality of families of curves parametrized by ${\mathcal{A}}_g(n)$

We prove that for every integers $g,h\ge 2,n\ge 3$, for all but finitely many prime numbers $p$, for every field $k$ of characteristic $0$ or $p$, every separable family of smooth projective curves of genus $h$ over ${\mathcal{A}}_g(n)\otimes k$ is isotrivial. To prove this, we compute the common vanishing locus of the absolutely logarithmic symmetric forms on a smooth complex algebraic variety whose universal covering is biholomorphic to an irreducible bounded symmetric domain of rank at least $2$.