I would go ahead and book something tentatively as there may be other graduations in the area that w

I wouldn't stay near USC and I'm not crazy about downtown either, for two women who are not familiar with the surroundings. I think I would stay over near the Grove and Farmer's Market, a mile or two longer by cab but worth it for the "safe feel" of the area. Sofitel, Farmer's Daughter cheap hotel deals at luxor in las vegas or Park Plaza Lodge Hotel...depending on Budget.
I would go ahead and book something tentatively as there may be other graduations in the area that weekend. Maybe the Park Plaza Lodge just to have something, you can always cheap hotel deals at luxor in las vegas cancel once you find something closer or that you like better, meanwhile you can breathe easier for the next few months knowing that you having SOMETHING booked.
Definitely cheap hotel deals at luxor in las vegas book something cheap hotel deals at luxor in las vegas ASAP. I would start in downtown L.A. with the two Marriotts (L.A. Live location is closer to the Metro, but 4th St. will be cheaper), the Omni, the Standard, the Bonaventure, Hotel Figueroa, Millennium Biltmore, Hilton Checkers. All of these are within a reasonable walking distance of the metro.
You may think the new Marriott is ugly, but the views from the guest rooms are spectacular and the L.A. Live location is probably the best area for walking around. Lots of restaurants, the Grammy Museum, frequent free concerts on the plaza, etc. The Marriott's ground floor lobby is a popular indoor gathering spot.