The Student Growth Percentile: Why Moderate-Income Districts Teach Just as Well
Part 2: Massachusetts' Student Growth Percentile metric proves that instructional quality is independent of district wealth.
The Massachusetts Department of Elementary and Secondary Education states: 'There appears to be little correlation between low income status and growth.' This official metric proves Hopkinton and Dover-Sherborn teach equally well despite $900K home price differences.
⚠️The Problem with Absolute Test Scores
When you compare school districts using raw MCAS scores or absolute proficiency rates, you're not measuring what schools teach—you're measuring what students bring.
A district full of children with highly educated parents, private tutors, and $2 million homes will post high test scores regardless of teaching quality. Those advantages are "wealth effects," not "school effects."
To conduct a valid comparison of school districts, you must isolate the variable of instructional quality from the confounding variable of socio-economic status.
Enter the Student Growth Percentile (SGP)
The Massachusetts Department of Elementary and Secondary Education (DESE) provides a metric specifically designed to solve this problem: the Student Growth Percentile (SGP).
What It Measures
How It Works
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A student takes the MCAS in 3rd grade and scores at the 60th percentile
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The following year, the SGP compares their 4th grade score to all other students who scored at the 60th percentile in 3rd grade
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If this student now scores better than 70% of their "academic peers," they have an SGP of 70
Key insight: A high-achieving student in Weston and a low-achieving student in a different district can both demonstrate high growth (e.g., a 90th percentile SGP), just as they can both demonstrate low growth.
🔥The Smoking Gun Quote
From MA DESE Official Documentation
— Massachusetts Student Growth Percentile Interpretive Guide
This official statement is the foundation of our entire analysis. It confirms, from the source, that a school's ability to teach and grow a student, year-over-year, is independent of that student's family income.
What This Means
The SGP data shows that teaching effectiveness is widely distributed and not concentrated in wealthy enclaves. In fact, 63% of all schools in Massachusetts have a median SGP between 40-60—clustering around the 50th percentile regardless of district wealth.
Real-World Application: Hopkinton vs. Dover-Sherborn
| Metric | Dover-Sherborn | Hopkinton |
|---|---|---|
Median Income | $250,000 | $130,000 |
Median Home Price | $2.4M | $925K |
Per-Pupil Spending | $23,969 | $16,571 |
MCAS Math | ~78% | ~72% |
MCAS ELA | ~82% | ~75% |
Student Growth | ~50th percentile | ~50th percentile |
The Critical Finding
Why This Matters for Home Buyers
| Question | What It Tells You |
|---|---|
❌ "What % are proficient on MCAS?" | What kind of students live in the district (wealth effect) |
✅ "What is the median Student Growth Percentile?" | How effectively the schools actually teach |
⭐ "What are college matriculation and AP pass rates?" | Whether schools successfully prepare students for the next level |
The Data Across All 9 Districts
This compression is only possible if instructional quality is relatively equal. If teaching effectiveness varied dramatically, outcome variance would be much larger.
The Final Implication
If Student Growth Percentiles show no correlation with income, and college matriculation rates vary by less than 5 percentage points, the logical conclusion is inescapable:
Bottom Line
For Home Buyers, This Means:
- ✅
Focus on outcomes (college rates, AP rates, graduation rates)
- ✅
Ignore prestige branding (U.S. News rankings reflect demographics)
- ✅
Evaluate value (outcomes per dollar of home price premium)
- ❌
Don't pay $700K-1.4M for identical teaching
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