Nothing about a 10/22 teaches bad marksmanship. Its the teacher and the shooter.
I never said it did. But there is a reason why single shot rifles are very popular for teaching young shooters who are going into hunting, especially if they are going into black powder hunting.
Now if you are just teaching a kid how to bang away at a tin can, then a single shot rifle is a waste of time. Just hand the kid one bullet at a time until he learns the operation of the gun, then let him go at it and bang away.