There were things you did in a battle that you'd never dare do in class. Vice versa was also true. Here they were sending stinging hexes and Stupefies back and forth. In a real battle, the spellset would be more varied, and more tricksy to work with, but the true task here was shielding and unshielding - developing a rapport with your partner (uhhgh, Malfoy), and giving as good as you got.
By the end of class, everyone was dripping with sweat (some people had absolute rivers running down their backs), and Harry could see some of the more beauty conscious girls trying to unsweatify themselves, to varying degrees of success. He had to give plaudits to Chang, who conjured up a scent so intoxicating that everyone stopped thinking about anyone's scent other than hers.
Naturally, Snape stopped this entirely with a glare, as Change wilted, finiting her spell at once. Which was a shame, because everyone (except perhaps Snape and his Slytherins) would prefer intoxication to the nose-bending stench of human sweat.
Before they could leave, Snape snarled, "Anyone who has not learned the Patronus should consider themselves considerably behind the rest of the class. I had planned to spend the next month working on this, so my office hours are open if any want to make use of them to learn this valuable - and newt-level skill. I will not waste the class's time with lessons that most people already have perfected." At least Snape had that going over Umbridge, his lessons were always carefully selected to get the best out of the class.
Harry packed slowly, managing to stay behind, as Snape turned to him, and asked, "I trust you have reason to waste my precious time between classes, Potter? Rather than say, coming to see me during my office hours?"
"I had detention the last two nights." Harry said, meeting his eyes, "The second because you weren't there the first. And another one for tonight."
"Ah, one of my loyal followers, no doubt. Draco Malfoy?" Snape drawled.
Harry met that with a simple nod, saying, "I think I understand, now, why you wanted an illicit club... there's plenty of folks that are happy to get one over on you. Makes them try harder, I think."
Snape smirked smugly, and continued, "And a secret that needs to be kept can be much more easily kept from more people, not less. If I'd done half the mixing of Slytherins and Gryffindors in class, there'd be twenty owls off to Death Eaters and their pestulent ilk."
"What else?" Snape prompted.
"It's harder to shrug off and not study when it's your friends - people what didn't have to do it - teaching." Harry said decisively.
"That'll be why Ron Weasley has improved so much over the last year and a bit, no doubt." Snape said smoothly.
Harry nodded, and asked, softly, "Can you cast Unforgivables in the Room of Requirement without being reported?"
Snape sighed, cradled his head in his hands, "I don't even want to know why you're asking that." Snape saw Harry's mouth start to open, and viciously cut him off, "The answer's yes, by the way. If you abuse the privilege, I'll find out, and you'll pay." That last bit was said with such assurance that Harry didn't for a second doubt it.
"Are... are you okay, sir?" Harry asked, with a trace of a stutter.
In less than an eyeblink, Snape had his wand trained on Harry's eye, his entire form in battlestance, eyes narrowed as he growled, "Care to ask that again?"
Harry straightened, himself, into something that would naturally have fit a military man about to salute. "No, sir." He felt, at least a little, relieved. Snape couldn't be too terribly hurt if his reactions were that fast, could he? Harry Hoped Not, at least. How like Snape to simply shrug off an expression of concern...
"Your detention tonight will be move to Friday." Snape said with a snap to his voice. "You are dismissed."
[a/n: Snape gives reassurance the only way he knows how, poor sod. "Yes, I am still capable of whupping your ass." This, my friends, is how Snape says, "I'm fine, thanks for asking. Don't Ask Again."
The whole thing about secret clubs making people study harder isn't my idea. It's rowlings, and really the only excuse she can give for how Ron Weasley is even competent enough to be in the same room as Inner Circle Death Eaters and expect to survive.