Alex Usher points to this World Bank report (89 page PDF) subtitled "A Systematic Review of the (Quasi‐)Experimental Literature on Outreach and Financial Aid." The authors "are exclusively concerned with outcomes of disadvantaged students" and focuses "on both enrollment in and completion of higher education." After a survey of the barriers faced by disadvantaged students, the report looks at specific interventions (their word, not mine) and assesses their success rate as revealed in the literature (the volume of which is in some cases unimpressive).
The results? "Outreach interventions targeted at students in high school or recent graduates seem to be a relatively cost‐effective tool to address inequalities in access to higher education, as long as the interventions go beyond providing general information about higher education." Support needs to be continued during the higher education period as well. Meanwhile, while "an early commitment of (needs-based) aid, while students are still in high school, leads to much larger impact on higher education access," by contrast, "merit‐based aid based only on academic results, without any assessment of students’ financial needs, seems to have no effect." Overall, "Interventions that combine early financial aid and outreach activities... seem to lead to large increases in enrolment rates, more consistently than either outreach or financial aid alone."