Translation:Arukh ha-Shulchan/Orach Chaim/554
Section 1 edit
On Tisha B'Av (The ninth day of Av) it is forbidden to eat, drink, washing, lathering/perfuming, wearing (leather) shoes and bedtime relations. It is forbidden to read Tora, Nevi'im and Ketuvim, and to learn Mishna and Midrash and Gemara, laws and stories, since it says: "The commands of hashem are proper, [and] bring joy to the heart" (Tehilim 19,9).
School children - are idle on it. It requires understanding why we prevent the children learning: If it is because of joy - they certainly have no pleasure in learning! The midrash on the verse "They traveled from the Mountain of Hashem" (Bamidbar 10:33) says this means like a child running away from school!
There are some who say that reason for the prohibition {of children} is because it gladdens the heart of the teacher (Taz subsection 1). This does not make sense, for how can we make all school study cease out of conccern for the joy of the teacher? Regarding the laws of mourning we have established that the when school teacher is in mourning - he needs to learn with the children, out of concern for neglecting the Tora of school students, as is written in Yore De'a Section 384. Furthermore, it is commonly known that the teachers suffer greatly in trying to explain things {to the students}, this is why their reward is so great as meantioned in Bava Batra (8b).
Section 2 edit
Furthermore, at the end of Taanit (30b) Rabbi Meir, and Rabbi Yehuda argue; Rabbi Meir permits to read and learn in a place where one is unfamiliar, for in these topics one will have discomfort trying to understand, however Rabbi Yehuda forbids it, and the law is according to Rabbi Yehuda, and according to the {textual} reading of the Rif and the Rosh, they also argue regarding school children, where {similarly} Rabbi Meir permits {study} and Rabbi Yehuda forbids it, and the law is like Rabbi Yehuda as well.
There is a need to understand Rabbi Yehuda's reasoning, where he forbids even in a place where one is not familiar, even thoguh this is definetely uncomfortable for him! There are those who write that his reasoning is afterwards he be happy when he finally understands (Taz subsection 2). However, if this is the case, by what logic does he prohibit children, for with children the idea of being happy after the fact is not relevant? Furthermore, How does Rabbi Meir permit {teaching - according to that reading}? Surely for the teacher there is joy at the time of teacching, acccording to the previous logic in section 1.
Section 3 edit
Therefore, it seems in my humble opinion the the matter is as follows: Certainly in learning of Tora there is Joy, as well as suffering while struggling in Tora, possibly even more {than the joy}. For there is no limit to the amount of depth in our holy Tora.
Regardless, the essential act of dealing with Tora, gladdens the Jewish Soul, even if there is suffering and struggle in the moment of learning. So Rabbi Meir supposes that for a new topic, and similarly for children and their teachers, since there is some discomfort at the time they are doing it - it is permitted to teach. Rabbi Yehuda feels that the suffering does not compare to the essential spiritual pleasure of learning the Tora of Hashem, even though one is not pleasuring the body in actuality, regardless, the Jewish Soul is gladdened with this.
This like the Rambam ob"m explains in chapter 2 of {hilchot} Gerushin in the matter of "Coercing him until he says 'I want it'", since in truth each and every Israelite soul wants to perform the wishes of his maker, even though the body does not.
Regardless, it is permitted to learn gloomy topics, for the soul also suffers in the troubles of Israel. Therefore, even though there is an element of Tora and Simcha in sad topics, nevertheless the suffering overpowers the joy, and therefore it is permitted, but the struggle of the learning itself, does not cancel out the Joy the Soul expeiriences from learning of Tora.