Education specialists are strongly advocating the use of collaborative teaching (co-teaching) approaches to facilitate the inclusion of pupils with Special Educational Needs (SEN) in mainstream settings. The current study analysed the impact of co-teaching on the numeracy attainment of students in two, mixed-gender Irish primary classes (N = 45; 4th Class: 9-10 years old) where there was a high incidence of SEN (e.g. autism, specific learning difficulties etc.,). After participating in co-taught numeracy classes for seven months, a standardised Mathematics test was administered. These scores were compared to the pupils' scores from the previous year when co-teaching was not the primary mode of instruction. A significant increase in pupils' overall standardised test scores was noted (p = 0.002). Scores for pupils with SEN in relation to numeracy (n = 12) also experienced a significant increase (p = 0.032) as did the scores for pupils without SEN (n = 30; p = 0.23). Based on these preliminary results, co-teaching appears to be an effective mode of instruction for meeting the needs of pupils with, and without, SEN in mainstream settings.