Yes, that would be the scene. Besides the tossing of the grenade, I can't think of any other time the cadets weren't treating the firearms with respect.
Regardless, I thought that this scene was forewarning the viewer to something much worse. Perhaps it was, although the particular cadet who asked for grenades wasn't involved.
I thought the opposite. I first saw this this in the theatre when it was released and the audience laughed at that scene. I think it was meant to be a lighter moment before things started going very badly. "And Shovel? Don't eat it!"
"'Extremely High Voltage.' Well, I don't need safety gloves, because I'm Homer Sim--" - Frank Grimes
All I got from that scene was a chance for Tom Cruise's character to show is A-hole attitude by ignoring the weaker cadet, along with the casual handling of weapons.