Research in inclusive settings is complicated by the nested relationships between the general education teacher (GET), the special education teacher (SET) and pupils. In this study, the impact of SET resource and selected variables of teacher competence (professional mathematical knowledge SET, attitude towards inclusion GET, classroom management GET) on the mathematical achievement gain of typically developing pupils (TYP) and pupils with intellectual disability (ID) was examined. Mathematical achievement was tested at the beginning of the school year (t1) and the end (t2) in 34 inclusive classrooms (sample ID: n = 42; sample TYP n = 525). IQ and gender – and the average mathematical achievement at class level in the sample TYP – were included as control variables. For pupils with ID, hierarchical regression modelling revealed that the mathematical knowledge at t1 explained most of the variance in mathematical achievement gain. For the group TYP, the results of a multi-level analysis showed that mathematical knowledge at t1, IQ and the average mathematical achievement at class level all had a positive effect on mathematical achievement gain. The more hours a SET was present in the classroom, the more the mathematical achievement of the group TYP increased. The other teacher competence variables had no apparent impact.