It looks like it's a multi-year, separate education track or program (both ChatGPT and Gemini's suggestions) for pupils with special needs. See for example below. So I guess if a school has 1a-9a and 1b-9b for 'normal' pupils, they may have 1z-9z for pupils with special needs. But I'm not an expert in education systems, which is also why I'm not posting this as an answer.
https://static.uvm.dk/Publikationer/2007/vikanjoproeve/kap06...'Specialklasserækken består lige for tiden af syv klasser, der dækker fra 0. til 9. årgang, i alt cirka 45 elever.'