There is a new Best Evidence in Brief with among others, this study:
An evaluation in the UK of the Education Endowment Foundation trial of Tutor Trust’s affordable instruction project found that low-cost tutoring in small groups increased math scores for disadvantaged students who are working below age-expected levels in math.One hundred and five schools in Manchester and Leeds with double the average numbers of disadvantaged students participated in the effectiveness trial of the Tutor Trust project from September 2016 until July 2017. The aim of the project is to improve the math achievement of disadvantaged students by providing small-group tutoring sessions with trained university students and recent graduates.Year 6 students (ages 10-11) who were struggling with math were selected by their teacher to receive extra support from Tutor Trust tutors, should their school be randomly allocated to the intervention group. The selected students in the intervention schools received 12 hours of additional instruction, usually one hour per week for 12 weeks, in groups of three. Students in the control schools continued with normal teaching. Achievement was measured using Key Stage 2 math scores (standardized tests in the UK).The report found that children who received tutoring from Tutor Trust progressed more in math compared to children in control schools (effect size = +0.19). Among children eligible for free school meals, the effect size was +0.25. There was also some evidence that students with lower prior achievement tended to benefit more from the tutoring.