That's true in my state, to weld on inside the boiler proper, a fitter and/or welder must hold an Oregon Class 4 boilermaker license, also to weld on power piping (steam above 15 psig) after the second root valve after the boiler proper, a fitter and/or welder must hold an Oregon Class 5 steamfitter license.
Welding on structural parts at a school, university, armory, (state/fed projects), etc. usually falls under local bldg codes inspection which means the welder must be currently qualified.
As an independent contractor I would not be welding on student chairs because of liability. Once I contracted to build aluminum bleacher railing and after checking with my bonding agent, had to take out more liability insurance for that particular job.