Found out that you need 80 Exposure to be able to get the miniskirt without underwear option, and those you would need for the final two rooftop scenes i believe.
Best way to make money is to 'sacrifice' a playthrough or two by spamming studying in the room, and then going for Doll/Broken Doll ending (you should get the option to make phone calls when at 100 stress after the first time), you should come out of it with some 20k or so which so far has been the quickest way to gain money that i have found, especially since Broken Doll goes straight to a premature ending.
There's a merchant in the gallery you end up in after the ending that sells the skill books, you can get some Exposure from rubbing one off on the table after class, walking up the left staircase to class in a miniskirt (with or without underwear) and some varied places in the school, but as you may have guessed it's not going to be even remotely enough to get to 80 in a week. You actually need to save up and load up with skill books specifically for unlocking that ending.
Keep in mind, stats get reset between playthroughs but you keep your money and items, so only actually use the books once you've saved up enough.