They do it beacuse they had a dream when they were in college training to be a teacher, to become a type of saviour to these kids, helping them to be someone, being a role model etc. When they start teaching and realise its not what they thought, they seduce a student as it gives them the saviour complex they desire, in a different, sexual form. Like they believe that they are giving this 15/16/17 boy something he couldn't get without them.