The fact that Pomfrey hadn't already swooped in on Stan like an angry owl suggested to Stan that she was overly busy. Wood probably shouldn't even be sitting up yet ("can't quite walk"- ha!), and yet she wasn't hexing him flat on his back.
"I'm a Slytherin," Stan pointed out to his friend with dry humor. "Of course I can sneak." He grinned a little at the dig on his own house. "Try this one on-"
With a quick incantation and a swish-flick of the wand, Stan levitated Oliver's belt, trousers, cast, and socks. He didn't lift them very high- for once thing, it would hardly look natural, and for another he didn't want to un-man Oliver, even if Wood was a prat. Still, if Oliver could move his legs just a little, it would probably pass a casual look.