BTW: It's very rare that people swallow their tongue (if you don't caunt a wrong guedel-airway insertion),
but a cause could be that the small piece of tissue under the tongue (we in The Netherlands call it the tongue-belt, but do't know how you call it in the US), that connects it to the bottom jaw tears off.
When that occurs, it's usually seen in soccer-matches when a player gets a very hard ball into his face, resulting in the tongue-belt breaking, not during a seizure.

Once again, don't place anything between their teeth, make sure they don't injure themselves to surrounding objects, and when the seizure is over, put the victim into the recovery-position.

That's about all you can do as a first-aid provider in a seizure... You could hurt yourself and the victim by inserting a bite-block during a seizure... If the victim does it himselve before a seizure, that's fine, but a first-aid provider shouldn't do it.

After the seizures end, place him into the recovery-position an when victim regains counciousness, you must have them lay down for about 10 minutes. If they stay uncounsignous for longer than 10 minutes, they have additional injuries or if a seizure is followed by another seizure (status epilepticus), you should call 911.