Determining disability prevalence is a growing area for population statistics, especially among young adolescents. The Washington Group on Disability Statistics is one source of reporting disabilities through functional difficulties. Yet, young adolescents self-reporting through this measure is in its infancy. The purpose of this study was to carry out an intra-rater test-retest reliability study on a modified set of items for self-reporting functional difficulties. Young adolescents (N = 74; boys = 64%; age M = 13.7, SD = 1.8) with special educational needs in Finland completed a self-reported version of the Child Functioning Module in a supervised classroom. The second administration took place two weeks later. Intraclass correlation coefficient (ICC) and Kappa (k) statistics were used to test the reliability of the items, and interpretation took place through Landis and Koch, and Cohen, respectively. The majority of items had substantial or moderate agreement, although there was only fair agreement for self-care (ICC = 0.59), concentration (ICC = 0.50), and routine changes (ICC = 0.54). Kappa statistics of behavior control were interpreted to be large (k = 0.65), and seeing (k = 0.49), walking (k = 0.49), and speaking (k = 0.49) difficulties were moderate. The majority of the items in the self-reported version of the Child Functioning Module can be used in a scale format, although some caution may be required on items of self-care and concentration when used as a dichotomous variable.