Marilee Jones, the dean of admissions at the Massachusetts Institute of Technology, has resigned after nearly three decades at M.I.T. after admitting that she had fabricated her educational credentials. School officials say she does not have even an undergraduate degree. Jones became well known for urging stressed-out students competing for elite colleges to calm down and stop trying to be perfect. The schools says a degree probably was not required for Jones’s entry-level job in the admissions office when she arrived in 1979. And by the time she was appointed admissions dean in 1997, she had already been in the admissions office for many years, and apparently little effort was made to check her credentials.