A proof of Wahl's conjecture in the symplectic case

Let $X$ denote a flag variety of type $A$ or type $C$. We construct a canonical Frobenius splitting of $X \times X$ which vanishes with maximal multiplicty along the diagonal. This way we verify a conjecture by Lakshmibai, Mehta and Parameswaran in type $C$, and obtain a new proof in type $A$. In particular, we obtain a proof of Wahl's conjecture in type $C$, and a new proof in type $A$. We also present certain cohomological consequences.