Mosad Chinuch Oholei Torah is a Chabad boys' school and yeshivah in Crown Heights, Brooklyn. The Rebbe's letters to it in the Igros Kodesh (roughly 5720–5730, 1960–1970) are mostly messages for its annual events and fundraising appeals, each drawing a lesson from the season.
Every Jewish child. The Rebbe stressed the vital role of Jewish education for every child and urged greater involvement in supporting it (IK 7630). Every Jew counts equally in receiving the Torah, so authentic Torah education must reach each child (IK 9143).
Education as a communal guarantee. Supporting Torah education is a communal responsibility rooted in our acceptance of the Torah (IK 8424); children are the true guarantors of the Torah, and supporting institutions like Oholei Torah during Sefirah prepares for receiving it (IK 9495, 9891). Every Jew shares the responsibility of spreading holiness, like the Kohanim (IK 9693).
Small beginnings, lasting results. Every positive act in chinuch, however small, can grow into lasting influence, as a seed becomes a tree (IK 8931). Genuine ethics between people are rooted in Torah values, which Torah-true education instills (IK 10144). Every day brings new opportunities for mitzvos done with enthusiasm (IK 10383), and the lesson of Shemitah — that true ownership belongs to G-d — applies everywhere (IK 10760).
Sources: the Rebbe's letters cited above.