The Black Leaves of Envy were told they would have to stop practising in a family garage after noise complaints from neighbours.
Foo Fighters frontman Grohl penned an open letter to the authority after the band contacted him.
He asked Cornwall Council to "reconsider the restrictions".
In the open letter, former Nirvana drummer Grohl says he started out practising in a garage and says music is a "wonderful, creative outlet for kids".
He said: "I believe that it is crucial that children have a place to explore their creativity and establish a sense of self through song."
Grohl also penned a separate letter to the band, giving them advice on soundproofing.
He advised the group to fill the garage walls with sand and cover the floor in gym mats.
Band member Adam Jones, 17, said: "It's just been surreal - I'm speechless."
"Seeing the Foo Fighters tag you in a post on Facebook was just incredible," he said.
It is crucial that children have a place to explore their creativity
The local authority said it had received complaints from a member of the public and told the band to keep the noise down to about 30 to 40 decibels.
A spokesman said it was "legally required to investigate once it has received a complaint about noise".
The spokesman said it had not told  the band to stop playing but had "advised that the sound levels are currently too high" and suggested installing sound proofing.
US rocker Dave Grohl has leapt to the defence of a teenage heavy metal band from Cornwall - after their local council said they were too loud.