Code reuse is a widely accepted practice in software engineering. Not surprisingly, the practice is becoming more and more widespread also in the context of computing education. This paper explores the use of code reuse in secondary education, focusing on the challenges that are connected to formative assessment. The paper presents the results of an interview study with 12 teachers teaching computing in secondary schools. Teachers have been asked about their attitudes towards code reuse in education, challenges with formative assessment and code reuse, and best practices to address the identified challenges. The study shows that attitude and approaches towards code reuse in the classroom span wide. Some teachers are open and positive to include it as a practice, while others are more restrictive. Most of the reported challenges revolve around the dilemma between learners reusing code to be efficient and create complex programs, but at the expense of adequately learning and understanding what they do and practicing general and specific programming skills. The paper also presents some lessons learned that can be directly relevant for educators. some lessons learned that can be directly relevant for educators.