Number One was only able to curse before the giant slime collapsed at him with its full weight. His back slammed against the wall upon impact and he was drowned in the gel-like substance.
Ah!
The female students exclaimed in worry and held their breaths as they watched Number One struggle to break free. He was clearly losing the battle and his lack of air only made it worse.
"It's over. He won't be able to get out on his own," Headmaster Blake said with a sigh. "He must have thought he can use his Soul Art from inside but the slime blocks any Soul Energy from getting in. He won't be able to recharge his Soul for another cast."
Headmaster Blake already figured out Number One's Soul Art and calculated how much Soul Energy he needed. Unless he had a Soul Art with a much lower cost, he couldn't escape.