BT BB 21a(38) :ואי איכא ארבעין מותבינן ריש דוכנא ומסייעינן ליה ממתא‏ : if there are forty (or more pupils) we install an assistant teacher and support him from at the expense of the town
See the entire entry for ytb vb. to sit ; to dwell; to stay, remain

BT BB 21a(38) :ואי איכא ארבעין מותבינן ריש דוכנא‏ : if there are forty (pupils) we appoint an assistant teacher
See the entire entry for ryš@dwknˀ n.m. assistant teacher (i.e. head of the platform)