The reason TV series episodes had teens listening/dancing to generic instrumental rock music was {1} to avoid paying royalties and {2} so in later years the episode won't seem quite so dated. Not sure but it may be possible that surplus stores sell dummy grenades but Gerald with his knowledge of chemistry made it active by adding explosive to a dummy grenade. As for the Mom & Step-dad being "inept" perhaps they overlooked the lad's emotional disturbance due to pride in his honor student status.
Review of The Grenade
