@Kayriel
MC doing that could be considered as an anomaly, as there are likely extremely few other 10 year olds that are capable of doing the same thing themselves.
So it's not necessarily wrong of him to accuse him of fraud, believing he had purchased the parts and wanted to hand them in and rise in rank quickly.
What's wrong is how instead of doing no confirmation whatsoever, he just outright went and accused him of fraud, and then threatened to expel him without any actual evidence.
When it was revealed Cain was a noble who became a noble by defeating orcs, who were either stronger than the lizardmen he supposedly defeated or around their level, two facts arose :
1) His strength is true
2) Nobles have a quasi-legal if not legal right to punish those that insult them if they so wish. And he had just spent the last few minutes vehemently insulting his power , character and honesty. Something many nobles would probably be very angry at.