Leaving Shul:<a href="#_ftn1" name="_ftnref1">[1]</a> 
It is forbidden to leave the Shul [once] the Sefer Torah is opened [until the completion of the reading, even if there are another ten people in Shul and he already heard the reading]. [In a time of great need<a href="#_ftn2" name="_ftnref2">[2]</a>, however], one may leave in-between Aliyos.
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<a href="#_ftnref1" name="_ftn1">[1]</a> Michaber 146:1; See also 149:1; Ketzos Hashulchan 25:11
<a href="#_ftnref2" name="_ftn2">[2]</a> M”A 146:2

Shulchan Aruch HaRav | Leaving Shul during the reading of Kerias Hatorah

Leaving Shul:[1] It is forbidden to leave the Shul the Sefer Torah is opened . , however], one may leave in-between Aliyos. ________________________________________________ [1]