As writing is a complex and resource demanding task, high-quality writing instruction is indispensable from primary grades on to support beginning writers in developing effective writing skills. Writing research should therefore provide teachers and schools with evidence-based guidelines for teaching writing in daily practice. In this respect, the present study first investigates the distinct and combined effectiveness of two instructional writing practices (i.e., explicit instruction and writing with peer assistance). Second, the present study aims to examine differential effects for students with different background characteristics (i.e., gender and general achievement level). Eleven teachers and their 206 fifth and sixth-grade students participated in the study and were randomly assigned to either one of the four experimental conditions (i.e., EI + IND: explicit instruction + individual writing, EI + PA: explicit instruction + writing with peer assistance, IND: matched individual practice comparison condition, and PA: matched peer-assisted practice comparison condition) or the business as usual condition. Multilevel analyses showed that EI + IND, EI + PA, and PA students outperformed the business as usual students. As to the distinct impact of explicit instruction, EI + IND students outperformed IND students at posttest, revealing the effectiveness of explicit instruction. As to the effect of peer-assisted writing, there were no significant differences between the individual writing conditions (EI + IND and IND) and the peer-assisted conditions (EI + PA and PA respectively).