it’s made clear that the girls can leave if they want, ...
Maybe on the Patreon page, but not inside the game :
"Have any of the girls every tried to run away?
...A couple have tried but they don't get far... There are guards at the station who would stop any student"
My apologies, I wasn’t clear in my previous post.
In the first punishment scenes with Debbie and Rachel, it’s made clear that the girls have a choice: she can “submit willingly” to her punishment, or she can refuse and be expelled. But being expelled means that should would likely be conscripted into the army.
So when I said the girls could leave, I didn't mean to suggest they could simply walk out the school gates and go home. I meant that they could choose to leave the school by being expelled. But that would have consequences (like conscription), and the girls would have to face those consequences. If any of the girls tried to avoid those consequences by running away.... well, that's what the guards at the station are for.
Aren't there any blueprints for a story where girls voluntary submit to spanking and punishment without means like blackmail, abuse of power or slavery?
Yes there are. It’s called 50 Shades of Grey. But having been forced to sit through one of the films, all I can say is that I’m really glad that Altos isn’t trying to recreate that film.
More seriously, almost by definition any story based in an institutional environment (school, prison, hospital etc) has to face that fact that there is a severe inherent power imbalance between the people running the institution, and those inside the institution. That means having to consider issues such as coercion and abuse of power, etc. I think it would be more unrealistic to ignore this and pretend it doesn’t exist. Far better, I think, is to address it in a meaningful and (semi-) realistic fashion that adds to the story.
Having said that, as you’ve already mentioned, there are lots of girls at this school who clearly enjoy (or don’t mind) what's happening to them e.g. exhibitionist Liz, kinky Amy, prefect Debbie, Lucy the maid. I suspect that’s about as close to vanilla consensual BDSM as you’re likely to find in a school-based spanking game.
Now, if you'll excuse me, I need to get back to spanking my safe, sane, consenting schoolgirls... and the hot female faculty staff.