There is a famous theorem due to J.-M. Fontaine, Il n'y a pas de variété abélienne sur Z (and independently by V.A. Abrashkin) that there are no abelian varieties over Z. I was wondering whether there is a function field analog of this result. More precisely: Let $k$ be a finite field. Is it true that there are no non-isotrivial abelian varieties over $\mathbb{P}^1_{k}$ with good reduction everywhere?

The case of elliptic curves is elementary. There are related results for families of curves but my question really focuses on the case of higher dimensional abelian varieties over $k(T)$.