Whiteboards can be flammable, but they are not an ignition source.
Here is a quick list of whiteboards and their flammability:
These are usually classed self-extinguishing. This means that they will stop burning once the source of the flame has been removed. Tosh & Leod are whiteboards are classed as this. They should not be burned on purpose, as plastics can release chlorine gas when burned which is dangerous.
These should not be flammable, the components of this are formed in high heat and therefore take a lot to of heat to burn. We do advise you speak with the manufacturer though as the backing board which are mounted to will be flammable.
These whiteboards will burn and may release toxic chemical when they do.