To expand my comment further, I do in fact have a copy of the 50+ page typewritten double-spaced document by Klaus-Dieter Schaper (dated June 1981). As usual this Diplomarbeit was not published, nor did Schaper himself apparently continue in mathematics. Though I'm not at all up-to-date on the problems surrounding Specht modules and decomposition numbers for symmetric groups, I might be able to answer specific questions about Schaper's actual work.

In any case, Schaper and a few others working on symmetric groups do include references to the influential early paper by Carter and Lusztig in Math. Z. 136 (1974): *On the modular representations of the general linear and symmetric groups* (available at the German archive GDZ). This paper preceded most of Jantzen's work, but it shows in a concrete way how to pass from the tensor representations of the general linear group (expressed in highest weight language) to the representations of symmetric groups on zero weight spaces. This connection is classical in characteristic 0, but leads to numerous complications in characteristic $p$ when this prime divides the order of a symmetric group acting on a tensor power of the natural module.

Jantzen showed how to start with a classical characteristic 0 module for the general (or special) linear group and then work with an explicit filtration of the resulting characteristic $p$ Weyl module, using his filtration coming from a contravariant form, etc. The resulting sum formula expresses formal characters using the dot-action of an associated (Langlands dual) affine Weyl group on the highest weight. In turn, by realizing an irreducible representation of a Weyl group on the zero weight space in characteristic 0, one gets by reduction mod $p$ a Jantzen-type filtrration in the resulting "Specht module" studied earlier by Gordon James. Then a sum formula, etc.

What you might find in Carter-Lusztig's explicit treatment in 4.2 is a transition from the affine Weyl group action to the symmetric group picture involving Young diagrams and raising squares. While the general theory has had a lot of further development, the ideas in Schaper's work (heavily influenced by Jantzen, of course) owe much to this earlier framework.

Matt Fayers might have more precise things to add. (Sorry for my earlier misspelling of his name, which happens to me all the time.)