Among individuals with autism spectrum disorder (ASD) who have a history of parent-reported developmental regression, or early skill loss, little is known about core-ASD symptoms (i.e., social communication, restricted/repetitive behaviors) and emotional/behavioral (E/B) symptoms during the school-age years. Study aims were to investigate whether early skill loss (i.e., ≤ 36 months old) was an indicator of heightened risk for later, school-based emotional and behavioral difficulties during the school-age years. Differences in teacher ratings for students with ASD from various age ranges (early elementary, late elementary/middle school, high school), with and without parent-reported developmental regression, were examined. Data were analyzed from teacher ratings for students with ASD (n = 951) on the Social Responsiveness Scale (SRS) and ASEBA Teacher Rating Form (TRF). Students’ families were participants in the multi-site Simons Simplex Collection (SSC), and students had study-confirmed clinical diagnoses of ASD. Overall, ANCOVA indicated few significant effects of developmental regression on teacher ratings of core-ASD symptoms or on ratings of internalizing or externalizing behavior problems, even when controlling for cognitive ability. These results, based on the perspective of teachers who are unaware of (i.e., “blinded” to) child regression status, were surprising yet suggest that early skill loss may not be an indicator of greater risk for some difficulties during the school-age years. Future work is needed to further investigate long-term outcomes among those with ASD who had an early skill loss, and teachers offer unique and important perspectives that may enhance future research in this area.