Does weed actually slow down your brain permanently?

I remeber as a kid my teacher told the whole class "if you smoke weed, your brain gets slowed down permanently forever" and I've known friends who smoked weed and they did seem kind of dumb even though they were smarter than me before they started. Like to the point where I could beat them in smash bros when I would bring in my switch, even though they used to beat my like crazy.

So is it true, or is there something else going on?