According to something I saw on QI once, yes you can.
Custard is a non-Newtonian liquid – this means it doesnt behave like most liquids do (with a constant viscosity) and sometimes acts like a liquid, and sometimes like a solid. If you apply a lot of pressure it acts like a solid, so running across custard fast would work. Walking slowly would not and you would sink as there would not be enough pressure.
I don’t know if you can watch youtube videos at school, but here’s a Brainiac experiment doing it