Brooklyn teen Quashon Burton thought bringing a laxative-laced cake to school would be "funny" - but he's not laughing now. After two teachers were sickened and he was hit with an assault charge, the senior is worried his whole future is collapsing like a half-baked souffle. "I'm pretty scared," Burton told the Daily News Thursday outside his home in Brownsville. "I didn't mean to have this whole thing blow out of proportion. I thought it would be a senior prank that everyone would think is funny." Burton, 17, and pals Tiara Peoples and Kenny Ramirez got the idea to...