  For a positive integer $N$ divisible by $4$, let
$\mathcal{O}^1_N(\mathbb{Q})$ be the ring of weakly holomorphic modular
functions for the congruence subgroup $\Gamma^1(N)$ with rational Fourier
coefficients. We present explicit generators of the ring
$\mathcal{O}^1_N(\mathbb{Q})$ over $\mathbb{Q}$, from which we are able to
classify all Fricke families of such level $N$.
