Start small

"Um, Professor," Mercedes asked. "Would you kindly strip naked?"

"Of course," Byleth said. She began to peel off her clothes one by one.

"So," said Byleth as she stood in front of Mercedes stark naked. "What exactly did you need help with?"

"I-I, uh..." Mercedes blushed at her professor's naked body. She had always liked girls and guys equally, but seeing Byleth in front of her, in all her glory...

"Would you kindly let me suck your nipples, Professor?" Merceds sueaked.

"Sure," Byleth said, setting her tits on the desk and leaning forward. Mercedes took one of her teacher's nipples in her mouth and began to suck it, while Byleth explained the lecture from today to her without batting an eye.