There's a cheat menu in there somewhere, right next to the journal / magic book.
In there, you can find "Direct input command," which will let you do pretty much anything if you know the right runes, or mess the game world up beyond comprehension if you do something silly.
There's also a time cheat sub-menu, but we don't need to go there.
In the direct input command box, you can type a variable to get its current value.
Enter "year" and get 2016.
Enter "age" and get 16.
Enter "vidage" and get 16.
Enter "year = 2015" or "year -= 1" and the year magically becomes 2015.
Enter "age = 15" and Sveta is suddenly a year younger.
Enter "vidage = age" and she doesn't look a day older than she really is (even if she's been going to bed with make-up on).
No promises that this won't mess everything up something awful, but if you make those changes as soon as the menu appears, before you've started any quest lines in progress so there's no "tomorrow" or "next Tuesday" or "three days from now" planned, it should work.
The graduation thing is year-dependent, not age-dependent, so you've just bought yourself an extra 366 days.
The "state" sub-menu lets you give yourself plenty of money for clothes, too, which is important when you're a 15-year-old girl and your inside leg or bust measurement is determined to occasionally get 2 cm longer overnight.
