sqad - as a male teacher of teenage girls, you accept what goes with the job.
That is - gorgeous voluptuous sexually alluring young women will develop crushes on you, and make sexual advances to you.
As a mature individual, and in a position of responsibility, you undertake to see these behaviours for what they are - initial sexual feelings for the nearest man outside family with whom a girl has regular contact. It is about exploring burgeoning adult feelings mixed up with a mass of contradictory hormones and confusion.
As a teacher, and a mature adult, the responsibility lies with the teacher to gently but firmly deal with these situtations with tact and diplomacy, and avoid anything getting out of hand.
To behave in any other way is a betrayal of your profession, your position of trust, and your presupposed ability to act like a responsible adult.
I can find no way whatever of justifying this man's behaviour - the girl behaved to type, and he exploited her through his own immaturity, and is now paying the price under the law.
Finding rational and reasonable answers that excuse his actions are virtually impossible, and tutting and saying he was 'human' or 'weak' simply doesn't cut it.