I guess they were using water soluble paints on pool since they intended to do a temporary art installation (like what they intended to do in chap 13). If they were using non water soluble paints they should've hosed their teacher with paint thinner instead.
I hope the boy dies and this becomes some kind of bruce wayne theater moment for the mc, although whats more likely is 'mc saves kid using magic', cause this is just your average, paint by numbers isekai.