Magic 'start' from the user.
Take the fire dash example you can see the 'magic line' start next to his feet before going on to make that spiral.
So for Kunon to make water ball inside he'd have to have sent his magic power in earlier or pierce through the barrier already.
The former isn't really possible since the teacher only reveal the test after putting up his barrier.
The latter would mean clearing the test already anyway.
That said, later on
when a kid challenge Kunon to a duel and start the fight with tidal wave
Kunon casually comment "no no, you must maintain control of your magic power after release or it'll get taken over" and turn the tidal wave into bubbles.
So 'hacking' magic is a possibility, but only against unskilled opponent which the teacher clearly wouldn't be.
EDIT: Also, the fire guy's name should be Hank (the little ` is 'side way' for nn, not the more 'vertical' so)
