Long story short, I'm an 18 year old who is headed off to college this fall (as a Finance major- go figure) and I'm wanting to get into investing. I've made an Ameritrade account, and I basically have around $250 bucks to play with. I'm not looking to get rich or anything, just maybe turn that $250 into $300 by June when I leave for college. First, what I've been doing:

I had a practice portfolio that follows the real market but uses fake money. An investing simulator I guess would be a good term for it. I've turned $10,000 into $16,000, but I know I don't have near that amount to invest in the real world, so I've tried a new strategy of investing in affordable stocks ($7-$10 per share), buying them in the morning s and dumping them after about two days, usually making around $15 profit each time.

Should I continue doing this with my real money? Is this a smart choice for someone with limited resources to invest, who doesn't want huge gains, just some spending money? And if not, what do you recommend?