Ask her if she'd like you to suck it.

"You know, sometimes it can feel really good to have your nipples sucked on," you say, "Would you like me to try that?"

"Yes!" she says with a red-faced smile, "but I thought we were just helping you feel less embarrassed!"

"Well, if I suck your nipples now, maybe I'll feel less embarrassed if Diana wants someone to suck my penis later."

"That makes sense," she says.

You bend down and gently run your tongue in a circle around her tiny, hard nipple. She shivers at your touch. You put your lips over it and begin to suck, first gently, then harder. She lets out a little moan, startling herself. You really want to just fuck her now, but that might be too soon for the class, and Diana would probably feel suspicious at your eagerness.

Instead you continue to suck on one nipple with your mouth, while your other hand plays with and squeezes her other breast. Her breath comes out in little burst...she's obviously super hot and turned on, but she isn't going to cum from just this. You sigh...I guess you'll have to wait a bit longer.