There is a yearly Master The Mainframe competition for students (but anyone can participate, get an account and do the exercises and go through the materials). It's fun. z/OS is completely alien for someone who grew up in a Windows/Unix ecosystem.
Also IBM is launching a new program for students called "IBM Z ambassadors" trying to reach students and evangelize about mainframes... I know it because Im currently an IBM Z ambassador.
https://zambassador.com/
There were bootcamps for AS/400 in the 1990s.
The AS/400 is a thing of beauty. It was ahead of its time and, in many aspects, it's still ahead of ours.