There have been cases of men being raped by a group of women - I remember seeing a thread on here about a case in South Africa a couple of years ago. By the sounds of it some men only need physical stimulation to achieve an erection; an erection does not automatically signify consent. The boy was 13; he could have been physically forced if he was small for his age. What's more likely though is that it started off being consensual, with the boy thinking he was so lucky...then the situation became overwhelming and he got trapped into it - forced, but only emotionally. Either way the teacher was horribly unprofessional, bordering on to paedophiliac (though that depends on how developed the boy was), and certainly very manipulative. She deserves a jail sentence.