There are basically several ways to find volunteers. It is certainly helpful to have a broader network that you can fall back on (e.g. contacts to the local university or to charitable institutions that have a pool of volunteers themselves). Sometimes there is also a self-organization of volunteers with a fixed contact person.
It can also be helpful to display flyers in public places (e.g. at the city administration).
Finally, schools can try to recruit volunteers themselves via their homepage.