With respect to engineers, versus field knowledge.
Both are important. FInding a balanced team is crucial too, but things have changed since the "old days".
Back in the day, field experience often won out over engineering calculations. Mostly we got away with it and mainly because everything was over-engineered to the point where it couldn't fall down whatever the work performed. There have been some spectacular fails, both here and in the UK (my home country). Bridges have fallen down, apartment buildings too. Sometimes the wrong materials were specified by the engineers, often the site workers simply didn't do what the drawings told them to do, and they buried their mistakes in concrete only for the evidence to emerge 30 years later.
What has changed is that many more exotic materials and designs are used, and even in traditional construction, materials have been pared as far as the engineering will allow. Skimping at the build stage will have faster and more dangerous consequences.
Recently we have authorized part two of the Keystone XL pipeline. It's safe, they say, with no danger of environmental disaster ... Yet Keystone 1 just sprang a leak dumping 200 000 barrels of oil on the ground. Safe.my.ass.
So yeah, we need the Coded and Certified Welders, but we also need to be training and certifying them correctly, and we need trained engineers to give them a spec, and for it to be accurate.
Back on topic ....
School bus chassis are massive, and massively strong. Car unibodies, not so much.
Welding on a chassis should present few real problems to a decent welder. It won't hurt the chassis and will take the weight ... if it's done right.
In the end, doing it right is really what matters. It will be vastly stronger than it needs to be and shaking certificates at each other won't help