Heh, funny comments...
Yeah, sensei is a bit creepy getting interested in a kid, but joke's on her.
She saw an immature kid and decided to tease him (no romance involved, just making fun of his reactions) and then when he showed he actually can get over his immaturity (occasionally) she started developing actual feelings.
Will she actually act on them is another point, she might "just" wait for him to turn 18.
And seriously? You read mangas and are shocked to see "onee x shota" content? Or is this only manga you read?