Tufts English major Alex Stein is well aware that he could be spending this winter break planted on a sofa in sweatpants or drinking beers to celebrate his impending graduation, or both. Instead, the 22-year-old senior will spend a week of his break in New York, working at Y&R advertising. “I know the job market is very tight for college grads right now,’’ Stein said. “Even if the job market wasn’t doing poorly I would still want to be out there. I don’t mind missing TV for this.’’ Stein is among a growing number of college students who are using a portion of their school break to take part in a so-called “winternship,’’ or winter internship. Facing a bleak job market and shrinking starting salaries, more students are doing winternships to bolster their resumes and, they hope, their chances at landing a job.