Is the teacher sinful if he contravenes the Maliki school of thought in teaching some Shar’i issues, knowing that there is supervision that mandates adherence to this school of thought?

There is no objection to limiting the teaching of Islamic sciences to the Maliki school of thought, even if it differs from you on some issues, especially if the competent authority stipulates or mandates it. This is for the purpose of organizing jurisprudence and facilitating its learning for students, and in obedience to the ruler in matters that do not involve disobedience. You are permitted to explain to the students that you are bound by the madhhab, and that whoever wishes to delve deeper should research its sources. Adhering to one of the four madhhabs is permissible, and people's approaches range from extremism to negligence and moderation. Whoever finds a stronger evidence may follow it. It is permissible for one capable of istinbat (deriving rulings) to take directly from the Qur'an and Sunnah. As for one who is not capable, it is permissible for them to imitate whomever they trust.