Try to restore them from your last (TimeMachine) Backup from the time before the reinstall: The lesson are installed in the System Library:
/Library/Application Support/GarageBand/Learn to Play/
or in your User Library in your home folder:
~/Library/Application Support/GarageBand/Learn to Play/
Copy them from there into the corresponding folder in your System Library.
Or you could try to download them again from the AppStore (Purchase History), see this Support Article: