This study investigated if the difficulty of instructional classroom English in primary teacher-guidebooks of English is adequately manipulated by learner-proficiency. Corpora of classroom English were compiled from 20 guidebooks from five publishers, approved following the 2015 Revised National Curriculum of Korea. Extracted materials from grades three and four were compared with those of grades five and six to observe variations in the difficulty. Coh-Metrix, a software application that computes an extensive range of measures on cohesion and language, was used for analyses. With evidence-based assessments on (psycho)linguistic features and patterns of classroom English, we report results both congruent and incongruent with the prospect that the difficulty should increase as learners become more proficient. Overall, although partial difficulty manipulations between the two levels were noted, inconsistent results and invariances were also observed, disclosing much room for improvement of classroom English in the guidebooks. Some implications toward teacher-guidebook development, particularly in its classroom English, are suggested.