There is a reason, and not just my sadistic glee at torturing characters behind it. Petra is just this terms special student and getting extra incentive with a chance at a reward later on. She just needs to think about it and get lucky.

Or willingly.
In fact if someone starts implying that, she may just beat them up.

Where Garth is talking to Petra, I actually read it to my mother and she started to giggle so I knew it was a keeper.

I had the entire first part thought out when I decided they'd throw away their first guide, I had to put Petra into an awkward position like that at some point.
I really didn't want them to be jerks to the boys, they're using them but not trying to be malicious about it, so while Marcus got the short end of the stick, the rest did ok, and they got to use Ella's money to get info they couldn't afford on their own, so it's a win win for everyone.
I also didn't want Petra and Ella solving everything. They're the big characters but the reason to have a group is to let everyone do something, and Clarice knew what she was doing having been around boys (who aren't her brothers) more than the others, along with a willingness to use her womanly charms.

How do you know the faculty hasn't realized this yet?
They rewarded the students who kept their keys and shared them with others. They specifically said how sharing or selling the keys was better than simply keeping any extras, and told the weak students they should have worked together or worked for a bully to get a key back.
They're very open minded about how people succeed, the key goal is quite simple, succeed no matter what. Some do that on their own, some do it through dominating others, some do it through allies. All of that is good.
So far Petra and her group are exceeding their expectations quite nicely. If they can teach them how to keep doing it, they've done their jobs as teachers of future overlords.

Like I said above, I had a lot of the basic idea all thought out. And since I really hate guided tours, I just skimmed it for the important bits rather than going and saying it "Oh look at the pretty flowers, aren't they amazing?" Blech.
So I was able to write it up in a few hours this afternoon.
And this is showing Ella's strong point, make allies, and make sure they know you can help them as long as they stay friendly. She's not the best planner in the world and she's a little too confident in her abilities but she knows how to make people join her side.

The boys were 100% me at that age. If I'd had a pretty girl I didn't know try this I'd have just gone with it and been too shy to speak to.

Don't forget Naomi, the spear maiden.

They have to start learning secret passageways somewhere, and they all have he basic layout of the school.

He found it amusing, which was why he teased them got photo's and made Elli jump through some hoops to talk to him. And he got more money from having Elli along then if he just had the normal boys. So he saw it as a win win. If the girls caused problem he'd just leave them in the dungeon or a maze or something.