Can a player react with Stupefy when an opponent plays a unit (Ravenbloom Student) that has no 'when I'm played' ability?

Ruling: No, you cannot react to units or gear being played. Even though playing a permanent enters the chain, no player receives priority so it resolves immediately, giving the opponent no window to respond.

Nuances:

• Units and gear do technically start a chain when played, but players don't receive priority during this chain
• You can only react to spells and triggered effects
• Both players in the scenario were partially correct: the unit does start a chain (Player B's point), but no priority is given (Player A's point)