Lisa and the others kept Vale's secret and didn't reveal Lotus' ability to help with their Spell Recording.
First of all, Vale mentioned that this type of help can't be done repeatedly since Lotus doesn't have an infinite amount of energy and is already cut off from the Spirit World. He decided to exaggerate Lotus' condition so Lisa and the others won't rely on her from now on.
It's not like he was being stingy but he knows that he has to limit this type of assistance as well. Learning how to properly record a spell without the assistance of Dark Spirit is what should be done since experience is what's really important in this type of thing.
After all, Vale wouldn't always be there for them.
***
On the 6th of June, Professor Russell Ravinie entered Class 2's room with a serious face.
He looked at Vale for a moment before he spoke with a solemn voice.