Rather than sitting behind her...I'm imagining sitting next to her. At the back of the class. Where no one can see what we're doing. And she unzips my pants and stuffs that mass of soft hair all over my cock and wanks me with it while class is still going on. She does such a good job I come all over her hair. She wipes my dick clean, and then coolly carries on as though nothing happened. She made me cum, so later after class I make sure I service her well and get her off nicely too. And of course I end with another dose of cumditioning.
To all the literary experts here, now that would be a nice story to write, wouldn't it?!