Struggling writers often need more instructional support than is present in commercially available process writing curricula. In this study, we employed a one-group, pretest-posttest design to evaluate whether 41 struggling primary grade writers who attended a university-based summer learning program would increase in writing ability, given a commercially available process writing unit differentiated to provide more support. A Teachers College Reading and Writing Project (TCRWP) unit was modified for use in the program: The content was streamlined, the volume of writing was reduced, the instructional explicitness was strengthened, increased feedback and student product goals were integrated, and the instruction was small group. We analyzed the data by students' entering level of intervention need (i.e., Tier 3, Tier 2) as determined by Written Expression Curriculum-Based Measurement (WE-CBM) Correct Writing Sequences (CWS) scores. Students who entered at Tier 2 level (N = 8) showed significant mean gains on the TCRWP On-Demand Performance Assessment and CWS assessment. Their pretest and posttest Test of Early Written Language-3 (TEWL-3) scores were comparable. Students who entered at Tier 3 level (N = 33) showed significant mean gains on all assessments. Analyses of progress relative to CWS end-of-year performance targets revealed five of eight Tier 2 students scored within Tier 1 at posttest. Three continued to score within Tier 2. Three Tier 3 students scored within Tier 1 level at posttest. Eighteen scored within Tier 2. Twelve continued to score within Tier 3. We discuss the findings in relation to the need for tiered support for writing instruction.