My only comment is regarding the particular letter that was quoted. Unlike a sichah, a letter is part of a correspondence. To understand it correctly, it is often necessary to know what the original question or report to the Rebbe was. Without that context, it is difficult to be certain of the Rebbe’s intent.
This letter contains two separate paragraphs.

In the first paragraph, the Rebbe acknowledges the school’s letter about its summer schedule and writes:
“I acknowledge receipt of your letter of 13 Iyar regarding the learning schedule for the upcoming summer… Certainly, before every Yom Tov they speak with the students regarding this, in addition to teaching the relevant halachos of the Yom Tov and customs in a way appropriate to their understanding.”

This appears to be discussing a different type of summer schedule than the regular school year, perhaps a schedule in which the students were not attending their usual classes every day. The Rebbe stresses that even under such circumstances, the children should be prepared for each Yom Tov. See also page 150 of the same volume and time period, where the Rebbe discusses this subject.

The second paragraph states:
“Regarding what you write that the younger students learn only until the afternoon break, apparently until 12:30, it requires clarification how they spend the rest of the day, which is most of it, especially since certainly some of these students are already of the age regarding which Chazal say ‘ספי ליה כתורא’…”

This paragraph appears to refer to a preschool or kindergarten, where most of the children were younger, while some had already reached the age of “ספי ליה כתורא.”

It is not clear that this paragraph is discussing summer learning at all and whether it’s continuing the same subject as the first paragraph. If the first paragraph is referring to a learning in the sumer when regular Yeshivah is not in session, then the second may be addressing an entirely different issue, namely that this particular preschool ended learning at 12:30 throughout the year. Without seeing the original letter sent to the Rebbe, it is impossible to know with certainty.

None of this changes the overall point. The Rebbe’s view that summer Torah learning should be as strong as during the rest of the year is stated explicitly in many other sichos and letters. For that reason, there is no need to rely on an interpretation of this particular letter when there are numerous explicit sources available.

Thank you again for raising such an important subject.
Y. Roness

P.S. Perhaps an important discussion that needs to be addressed is the practical steps to implement summer learning for children.