Software engineers are tasked with writing functionally correct code of high quality. Maintainability is a crucial code quality attribute that determines the ease of analyzing, modifying, reusing, and testing a software component. This quality attribute significantly affects the software's lifetime cost, contributing to developer productivity and other quality attributes. Consequently, academia and industry emphasize the need to train software engineers to build maintainable software code. Unfortunately, code maintainability is an ill-defined domain and is challenging to teach and learn. This problem is aggravated by a rising number of software engineering students and a lack of capable instructors. Existing instructors rely on scalable one-size-fits-all teaching methods that are ineffective. Advances in e-learning technologies can alleviate these issues. Our primary contribution is the design of a novel assessment item type, the maintainability challenge. It integrates into the standard intelligent tutoring system (ITS) architecture to develop skills for analyzing and refactoring high-level code maintainability issues. Our secondary contributions include the code maintainability knowledge component model and the implementation of an ITS that supports the maintainability challenge for the C# programming language. We designed, developed, and evaluated the ITS over two years of working with undergraduate students using a mixed-method approach anchored in design science. The empirical evaluations culminated with a field study with 59 undergraduate students. We report on the evaluation results that showcase the utility of our contributions. Our contributions support software engineering instructors in developing the code maintainability skills of their students at scale.