It sounds weird, but this is the sociological term. See the following "serious" examples:

Etiology of school backwardness
by J Konopnicki - 1969 - Cited by 1 - Related articles
School backwardness is one of the main social calamities of our time. ... School backwardness is a process which, if it is allowed to develop, ...
www.springerlink.com/index/V15277LUW4Q4H0N5.pdf

Data from the 2000 Population Census were used to construct two sets of multilevel logistic regression models to quantify the risk of school backwardness ...
blds.ids.ac.uk/cf/opaccf/detailed.cfm?RN=300674

Indeed, it is probable that general school backwardness is more frequent in emotionally maladjusted children than is backwardness in reading alone. ...
books.google.com/books?isbn=0521144418...

... using the individualized care project, should avoid this absenteeism, which is responsible of school backwardness and difficulties in choosing careers. ...
www.ncbi.nlm.nih.gov/pubmed/15301809

These courses covered a wide range of childhood disorders such as developmental disorders, emotional and behavioural problems, school backwardness, ...
www.emro.who.int/publications/emhj/0203/16.htm

... were the most significant factors accounting for school backwardness and truancy.35 In order to remedy these problems, legislation was enacted allowing ...
books.google.com/books?isbn=0765804565...

the role which physical defects play in inducing school backwardness, as compared with other factors relating to the child's actual school history. ...
www.jstor.org/stable/1077400

Just another idea. I think, from all the references I've seen, that "atraso escolar" means the same in Mexico as in Spain; it doesn't mean that the schools are backward, but that the pupils are falling behind the level they should be attaining: failing to keep up, and indeed having to repeat failed years of study. In discussions of this issue in the UK, at least, the expression "falling behind at school" seems to crop up all over the place. It seems to me a natural expression in the context.
I think "educational backwardness", suggested on an earlier occasion in KudoZ (http://www.proz.com/kudoz/spanish_to_english/education_pedag... , is also worth considering, but to me it tends to imply that the educational system itself is backward in that area, and I don't think that's what they're saying.

"El síndrome del atraso escolar y el abandono del sistema educativo
Se recogió una muestra de 1 840 niños en edad escolar distribuidos por igual entre estudiantes de escuelas primarias que cursaban el grado siguiente a aquél en que se encontraban el año anterior. y niños que estaban repitiendo el mismo grado o habían abandonado la escuela. El diseño muestral abarcó distintos tipos de escuelas primarias en tres entidades federativas. Se hicieron análisis multivariados para determinar las características que distinguen a los alumnos regulares de los repetidores y desertores. Se observó también una muestra de grupos escolares, con el fin de identificar algunos efectos que produce la interacción entre maestros y alumnos, en el avance escolar de estos últimos. Se llegó a diversas conclusiones que pueden ser utilizadas por los planificadores del sistema educativo con el fin de mejorar la eficiencia de 181 escuelas primaria del país."
Article on Mexico in Revista latinoamericana de estudios educativos,
http://dialnet.unirioja.es/servlet/articulo?codigo=2237855

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Note added at 3 hrs (2011-01-18 00:14:40 GMT)
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Wendy: on more careful reading of your question I see you had already thought of almost exactly this phrase in the first place, before asking, so I take it you were not happy with it. However, I urge you to reconsider. I think it would work fine in your context, and is the most natural way of expressing the idea of not just achieving poor results, but failing to keep up at all, leading to repeating years and eventually dropping out. Or perhaps, after all, the dreaded word "failure" should be brought in here!

a "late developer" is a pupil who only reaches the normal standard for his or her age late in his or her school career... my mother was one of these.

cheers

I've never heard or used "backwardness" when referring to education as it involves a judgement on the one hand and fault on the other, which would never be acceptable in the UK education system. This is perhaps an EU, NGO (Unesco) or N. American way of putting it.

I like jude69's 'poor educational standards', which is what I'm getting at too.