Is the teacher's salary considered permissible (halal) or forbidden (haram) if he is unable to control the students during the lesson? And what is the ruling on deducting a portion of his salary (the "Mardoudiyah" point) for his refusal to attend national school events that include music and photography?

The teacher should strive to silence the students in order to explain the lesson, using available means, and reporting those who do not comply to the administration. If he does that, then there is no blame upon him and his salary is permissible. As for his abandonment of musical concerts, he is praised and rewarded for it, and he must endure with patience and seek reward if any harm results from that.