I hope your exam preparations are going well and b'hatzalacha on all of your final exams!
I wanted to share that I found the source for Rabbi Bernstein's teaching that one is permitted to act angry but one must not be angry in one's heart.
Mesilat Yesharim 11:143 Our sages of blessed memory warned us to not get angry even for the sake of a mitzva (see Shab.34a, 105b), not even a teacher with his student or a father with his son. This is not to say he should not chastise them, rather he should surely chastise them, just not out of anger, but rather, with no other purpose than guiding them along the right path. Any anger that he shows to them should be only of the face not anger of the heart. Shlomo said: "Be not quick in your spirit to be angry [for anger rests in the heart of fools]" (Kohelet 7:9) and it is written "For anger slays the fool" (Iyov 5:2), and our sages of blessed memory said: "in three ways a man is recognized: through his cup, through his wallet and through his anger" (Eruvin 65b).
There is great joy in finding sources and how the verses of the Holy Teachings of the Torah echo each other like a beautiful symphony. Are we not the blessed of the nations to be able to study Torah?
Also, I pray for speedy middot correction of anger issues!