Buddha Jayanti tipitaka sinhalese version has the word, Santhāgāra not sandhāgāra (BJT Diganikaya I, pg 362.) The meaning of the word Santhāgāra; [Sk. sansthāgāra] a council hall, a mote hall (PTS dictionary).
Addasā kho āyasmā ānando sambahule licchavikumārake santhāgāre upāsanaṃ karonte, dūratova sukhumena tāḷacchiggaḷena asanaṃ atipātente, poṅkhānupoṅkhaṃ avirādhitaṃ
He saw several Licchavi youths practicing archery. They were shooting arrows from a distance through a small keyhole, shot after shot without missing (Ven. Sujato).
(Ven. Sujato missed the word. ) Vāla Sutta SN 56.45
santhāgāre - In the training hall (Ven. Bhikku Bodhi)
santhāgāra in the sutta seems to be a school.
tathā hime abhiññātā abhiññātā licchavī santhāgāre sannisinnā sannipatitā anekapariyāyena buddhassa vaṇṇaṃ bhāsanti, dhammassa vaṇṇaṃ bhāsanti, saṅghassa vaṇṇaṃ bhāsanti. (Besajjakkhanda)
Sīhasutta also shows us the same; a meeting hall.
Tena kho pana samayena cātumeyyakā sakyā santhāgāre sannipatitā honti kenacideva karaṇīyena. MN 67
Here the word used in a meaning of a meeting hall and this seems to be a place where educated people met and discussed their views and shared their knowladge.
Translating the word to meeting hall is correct. However, being secluded in a meeting hall is not possible.
Therefore, the word here used with a meaning of some kind of a temple as in Kandaraka Sutta.
He has a new temple built to the east of the city. He shaves off his hair and beard, dresses in a rough antelope hide, and smears his body with ghee and oil. Scratching his back with antlers, he enters the temple with his chief queen and the brahmin high priest.
So puratthimena nagarassa navaṃ santhāgāraṃ kārāpetvā kesamassuṃ ohāretvā kharājinaṃ nivāsetvā sappitelena kāyaṃ abbhañjitvā magavisāṇena piṭṭhiṃ kaṇḍuvamāno navaṃ santhāgāraṃ pavisati saddhiṃ mahesiyā brāhmaṇena ca purohitena (MN 51).
MN 94: Ghoṭamukha Sutta also shares the same passage.
The temple here is a place built to carry out offerings and sacrifices such as assamedha, purisamedha.
Assamedhaṃ purisamedhaṃ, sammāpāsaṃ vājapeyyaṃ niraggaḷaṃ;
Mahāyaññā mahārambhā , na te honti mahapphalā.