  We elaborate and extend the method of Wronskian differential equations for
conformal blocks to compute four-point correlation functions on the plane for
classes of primary fields in rational (and possibly more general) conformal
field theories. This approach leads to universal differential equations for
families of CFT's and provides a very simple re-derivation of the BPZ results
for the degenerate fields $\phi_{1,2}$ and $\phi_{2,1}$ in the c < 1 minimal
models. We apply this technique to compute correlators for the WZW models
corresponding to the Deligne-Cvitanovi\'c exceptional series of Lie algebras.
The application turns out to be subtle in certain cases where there are
multiple decoupled primaries. The power of this approach is demonstrated by
applying it to compute four-point functions for the Baby Monster CFT, which
does not belong to any minimal series.
