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.

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

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'm assuming you mean other than things like the one cadet reclining in bed, reaching up and grabbing a grenade and tossing it to the requestor? It's been a long time since I was in the Army, but still lol
Introduce a little anarchy. Upset the established order

My only disagreement here is that I didn't see any sort of casual or dangerous attitude displayed by any of the cadets regarding their firearms throughout the film. All were rather professional and cautious, and appeared to have good trigger discipline throughout the film.

It was to show the cavalier attitude the children had towards the handling of weapons.
Introduce a little anarchy. Upset the established order