Formative assessment is considered to be a promising teaching practice for promoting teaching and learning processes. The implementation of teaching practices into instruction involves intervening with a learning environment that is characterised by certain features of instructional quality. Our study aims to contribute to the understanding of formative assessment by analysing the interplay between a formative assessment intervention and aspects of general instructional quality. In a quasi-experimental study design, 15 teachers participated in a control group (n = 361 students) and 20 teachers in the intervention classes (n = 498 students) implemented a curriculum-embedded formative assessment tool in their ninth-grade mathematics classes. No effects were found for the intervention on the assessed aspects of general instructional quality (process-oriented instruction, teacher-student relationship, effective use of instructional time). However, multilevel regression analyses revealed an interaction effect between the intervention and process-orientation and the effective use of instruction time. Our findings suggest that implementing formative assessment tools do not seem to suffice regarding changes in general instructional quality, but that an intervention with detailed material and guidelines can counterbalance effects of instructional quality, fostering students' achievement in classes with lower degrees of process orientation and a less effective use of instructional time.