There is good research on policy, but because so many individual policies are in play at any given moment, it is very difficult for policy researchers or policy makers to figure out the effects of anything.
Proven tutoring models of all types should be a standard offering for every school. More research is always needed to find more effective and cost-effective strategies. But there is no reason whatsoever not to use what we have now.
Like many things, there may be a time and a place for using subsidies. But they need to be better targeted. And, often, there will be better alternatives. Alternatives that do a better job of protecting the poor.