The purpose of this study was to determine the feasibility, acceptability, and potential efficacy of a school-based physical activity program delivered during school sport time among adolescent girls from low income predominately linguistically diverse backgrounds in New South Wales, Australia. Using a 3-month, 2-arm, parallel-group pilot RCT design, 38 adolescent girls (Year 11) were recruited to participate in the program and randomised into intervention (n = 17) or control groups (n = 21). The intervention program aimed to increase physical activity by improving enjoyment, physical self-perception and perceived competence. Baseline and follow-up (12 weeks) assessments included enjoyment of physical activity, physical self-perception, and objectively measured physical activity during school sport sessions. Process data were collected through observations of lessons, attendance records, and interviews with participants and staff. Recruitment (63%) and retention (68%) goals were less than anticipated but similar to other studies. Participation was higher for the intervention (72%) than the control (60%) group and the intervention group reported high levels of satisfaction with the program. At follow-up, girls in the intervention group, compared with the control group, showed greater improvement in their enjoyment of physical activity during school sport (adjusted mean difference = 3.8, 95% Confidence Interval [CI] −2.4, 10.1; Cohen’s d = 0.42 standard deviation units) and body image (adjusted difference mean = 1.0, 95% CI −0.4, 2.3; d = 0.50). There was a smaller decline in participation in physical activity during school sport (adjusted mean = 13.6, 95% CI −21.8, 48.9; d = 0.24). This study highlights major barriers confronting adolescent girls’ participation in school sport. Some of these include teacher attitudes and support, activities and programming, purpose and distinction, and student input. Negotiating these barriers and overcoming them in a school setting appears feasible with support from the entire school community.