He lets it play out.

Too stunned to try another test, he waited until class ended, and Byleth assigned them a small writing assignment regarding tactical uses of their weapon of choice. Everyone else filed out, and Sylvain waited nervously.

Byleth approached him, and grabbed his hand instead of starting to lecture him, and practically dragged him along to an empty room. Sylvain stood in shock as he followed along and she closed the door.

Giving him a smile that seemed out of place, what with how stoic she tended to be, she dropped to her knees and began to work on his pants. Sylvain was in disbelief. The Note actually worked, there was no other explanation for this.

She pulled his pants down, and his dick sprang free. She began to stroke it with her hand until it was at full mast, which, given how aroused he already was, only took a few seconds. With an elated look in her eyes, she engulfed his shaft in her mouth, and slid her head down it until her nose was touching his lower abdomen.

She then began bobbing her head on his shaft, fondling his balls as she worked on it. He gasped at just how good it felt. While he was no virgin, he'd never managed to get a girl to give him head before. This, combined with the arousal of the situation, made him feel amazing.

He put his hands on her head and began to thrust his hips gently. She moaned around his cock and moved her hands to his thighs to steady herself. It wasn't long before he was fucking her face.

After several minutes, his cock was throbbing, and he moaned out as he released his seed, filling her mouth. She swallowed it all without complaint, and then she pulled off, panting, and looking up at him with just a hint of lust.