Once you make sure that your rim strip is covering all of the sharp-edged spoke hole inside your rim, and you're ready to replace the tube, try one of the tubes that come with sealant inside. Yes, they're a little heavier and they're kind of silly, but they'll work great for a commuter bike back/forth from class. Make sure that there's nothing still stuck through your tire by CAREFULLY running your fingers all around the inside of the tire while it's completely removed from the rim, or use a cotton ball to do the same. If there's a sharp object embedded in the tire you'll find it this way. Doubtful that it's your issue, but might as well rule it out. Good luck, flats, suck.