ＬＨＲ
ロングホームルーム- 1long homeroom ; occasional or periodic extra long registration class or assembly in a school (e.g. for activities not related to class work) Translated by Jiyomi
Other writings and readings
ロング・ホームルーム
noun (common) (futsuumeishi)
JMdict entry 2146610. Source and licence.