The OECD's PISA project produces a lot of reports that might help. For example, there is:

OECD (2016). Low-Performing Students: Why They Fall Behind and How To Help Them Succeed.

http://www.oecd.org/edu/low-performing-students-9789264250246-en.htm

Dear Frank: You already got some answers to your question and I don't know a better answer but I do have a story that you might find interesting. While taking a walk one day, I went by a garage sale and purchased an old (1936) physics textbook for high-school students. This book is called "New Elementary Physics" by Robert A. Millikan and Henry G. Gale . This book is old enough to believe that a hydraulic lift (used everywhere nowadays) is a brilliant invention (it actually was a great invention).  What absolutely amazed me was the level of sophistication that this book expected from high-school students. It discussed topics, such as surface tension on fluids and why this makes fluid surfaces have certain shapes, that I would think is a more appropriate topic for a graduate student. That is only one example. There are many others. This book convinces me that people worked a lot harder in old days for an education.

One of the ways which might be useful is to support any "theoretical" statement by sufficiently many and appropriate examples, rather than insisting on the proof of the statement. This method works especially when the proof is long and difficult. Such proofs can be simplified by restricting yourself to a particular significant case.

Improving achievements in math is not immediate. it is a long process that begins,basically, with  preparing qualitative teachers ,a major axis of the teaching process:teachers with good  TPACK knowledge (Technological,Pedagogical And Content Knowledge).-"A good captain is needed to navigate the ship"

At home, parents are asked to implant positive thoughts about math, in their children's brains.

 supplying suitable budgets, for funding and developing math teaching and learning projects in the society.