This is a common misconception that is frequently associated people who suffer from epilepsy. It is impossible for a person to swallow their own tongue because tongues are rooted to the floor of our mouths by a segment of mucous membrane called the lingual frenulum.
Don't place an object behind the tongue to prevent someone from 'swallowing' their tongue. This only serves as a choking hazard. In reality, choking is a main risk with this sort of episode - a seizure victim should be placed on their side in order to prevent saliva or other bodily fluids from blocking their airway.
(Sources: 1, 2)