As computing skills become necessary for 21st-century students, infused computational thinking (CT) lessons must be created for core courses to truly provide computing education for all. This will bring challenges as students will have widely varying experience and programming ability. Additionally, STEM teachers might have little experience teaching CT and instructing using unfamiliar technology might create discomfort. We present a design pattern for infused CT assignments that scaffold students and teachers into block-based programming environments. Beginning with existing code, students and teachers work together 'Using' and comprehending code before 'Modifying' it together to fix their programs. The activity ends with students 'Choosing' their own extensions from a pre-set list. We present a comparison of two implementations of a simulation activity, one ending with student choosing how to extend their models and one having all students create the same option. Through triangulating data from classroom observations, student feedback, teacher interviews, and programming interaction logs, we present support for student and teacher preference of the 'Student-Choice' model. We end with recommended strategies for developing curricula that follow our design model.