Ask her to continue.

As Diana slows her pumping, you turn to whisper in her ear, "This feels great, please don't stop."

"I've got to keep the class progressing," she whispers back, "what's it going to teach them if I just keep tugging you off?"

"I don't know! They could see what I look like while I orgasm."

"That's not till the end of the class," she says, exasperated, "Most of our lesson plan requires you to be hard. I'll give you a few more strokes but then we have to continue."

She starts pumping on your cock again, and you thrust in earnest, trying to make the most of whatever time she gives you. You can tell her heart isn't in it, though, and pretty soon she tapers off.