I've always found the concept intriguing...
Does anyone know a manga about a school where boys are trained to be slaves, or masters?
(Or if not a yaoi manga, is there a manga at all with this plotline?)

There's Ai to Yokubou wa Gakuen de where boys are training to be sex workers and hosts.... I like the art quite a bit and despite the story premise this is not just hard p0rn with no plot... This one has story and romance as well. Highly recommended.

This one is not SM perse, but there's Oshioki Gakuen that tells a story of an unreal, perverse teacher and highschool. This one is just p0rn

For a non-yaoi one, I think there's Nana to Kaoru that somehow 'teaches' about SM. It's ecchi though.

Quote from mizunosaki

There's Ai to Yokubou wa Gakuen de where boys are training to be sex workers and hosts.... I like the art quite a bit and despite the story premise this is not just hard p0rn with no plot... This one has story and romance as well. Highly recommended.

This one is not SM perse, but there's Oshioki Gakuen that tells a story of an unreal, perverse teacher and highschool. This one is just p0rn

For a non-yaoi one, I think there's Nana to Kaoru that somehow 'teaches' about SM. It's ecchi though.

Thanks~ Actually, I've read Oshioki Gakuen. It's interesting, but it's too weird o.o
(and like you said, way too unreal ._.)