שלום וברכה!
[ו' שבט תשכ"ד]
... שילמד תנ"ך וכיו"ב לשומעים שישבו בגילוי ראש וכו' - מובן שאין נ"ל כלל. ובפרט שכו"כ מהם ישבו כהנ"ל לא מתוך אונס וכיו"ב. וד"ל.
נדפסה בלקו"ש חכ"ג ע' 493.
Shalom u'Vrachah!
... To teach Tanach and similar subjects to listeners who are sitting with uncovered heads, etc. – it is clear that this is not appropriate at all. Especially since many of them sit as described above not out of compulsion or the like. And this should suffice.
Summary
The Rebbe teaches that Torah learning, especially Tanach, should be conducted in an atmosphere of respect, including proper head covering, and it is not fitting to teach those who disregard this without necessity.