joseph koetters has been sued by an unnamed marlborough student , 30 .
lawsuit alleges that koetters took part in a year-long sexual relationship with a 16-year-old student , which began in the 2000-2001 school year .
lawsuit also claims unnamed teachers and school officials are held responsible for not ending the relationship .
woman came forward after another former student wrote an online essay accusing koetters of acting inappropriately toward her .