I love the fact that we get to have input to your books! It’s quite the privilege, actually. Just had to say that hehe.
Anyway, enough with the gushing :o) Love the opening. Illuminorbs sound amazing. One thing I would think about though is the exclamation mark after “This was so frustrating!” Maybe make Tilda do something to vent that feeling – pound her leg with her hand, make an angry noise, whatever is appropriate. Make the, er, make the action the punctuation … or something …
I did feel that the passage after that also was a little long. I wanted it to pause around this point:
“Surely she was using up all her available power, just practising this one thing. And if she failed to manage to produce an orb, what then?”
Then when you say “Silviu had made it quite clear”, maybe S. can make it clear then. Have the disappointment happen then, perhaps. It just skirted a little close to clock-stopping exposition for me. If Silviu (cool name btw) expresses his* disappointment then, it can make some challenges for the MC plus flesh out Silviu.
Just some thoughts anyway. Hope the help you out :o)
* Incidentally, this sentence: “and Silviu had made it very clear that she would not be allowed to progress further towards becoming a functioning mage until she succeeded.” made me think S was female. Had to do a double take when I realised that wasn’t the case.