Stan needs to initiate. Remember the hidden camera? Give them a show... Stan uses blackmail to get his way. To start the questline, have Serena go to school horny in a short skirt outfit (Black Suit, Revealing Suit) and masturbate during the lesson (self-study; if she's been horny enough, she has probably hinted at wanting to masturbate but that her skirt is too long). Once she's done the deed, Stan has what he needs to get serious.
Gift suit:
The abandoned building was quite literally abandoned in development, it has no content. I tried so many times to get in there as well...
If memory serves, the next step for the GEM/photo studio quest is to have Serena dominate Pete. You need to ambush him outside the studio, the spot will be highlighted (sparkles). I forget if there is a trigger for it besides enticing Pete to fuck Serena in the school library and talking to the owner, but you can always check the Recollection Room for hints.