Marcus decide to take Lia and Nico to the mana supplying room during 'sightseeing' tour of the castle.
His reason? Childishly wanting to show off supplying the barrier basically.
Needless to say, the adults quickly found out and he got massively scolded for it.
One thing that was brought up out of this was that he seem too tired for merely supplying the mana. He should have the mana capacity for it already so this suggest he's terrible at mana control.
Solution: He now must attend the children's mana control lesson

(he's 20 btw)
They can still go back to mana supply room if they want to, but they must first properly inform the adults of their plan to do so.