You are absolutely right to resist this teacher's behaviour, which is wicked and wrong. But you must not punish yourself by missing your classes. Please confide in a close friend and make sure that if the teacher asks you to stay behind, the friend stays too. If the teacher insists that you stay alone, make sure that the friend remains close enough to hear. If the teacher repeats this behaviour, then scream, and let your friend be witness to what is happening. Refer the matter to the headmaster immediately. Alternatively you could confide in a female teacher, or tell someone in your family.