Can starting daycare or school early cause ADHD-like symptoms that get misdiagnosed as ADHD?

It's the second, more often, an adjustment reaction rather than a new condition, though it deserves a careful look rather than being dismissed. Starting daycare or school is a major transition, and some children show temporary attention or behaviour changes as they adapt to new structure and expectations, which can resemble ADHD symptoms without being a chronic condition. Genuine ADHD requires symptoms to persist across settings and over time, not just during an adjustment period, which is why clinicians look at duration and consistency rather than diagnosing off a rough first few months. References: https://www.caddra.ca/wp-content/uploads/Canadian-ADHD-Practice-Guidelines-4.1-January-6-2021.pdf

My daughter had a rough first month of kindergarten that looked a lot like this. Symptoms faded almost completely by month two once she settled into the routine. We've stayed close with a few other only-child ADHD families over the years and nobody's noticed any pattern either, for what it's worth. For what it's worth, that's two data points against the theory, not for it.