We have established a network of secondary schools in the Netherlands with students routinely measuring aerosol optical thickness (AOT) at two wavelengths with hand-held Sun photometers. Students have performed more than 400 measurements between January 2002 and October 2005 over more than 12 locations within the Netherlands as a contribution to Global Learning and Observations to Benefit the Environment (GLOBE). We have developed an improved AOT retrieval algorithm that accounts for the effective wavelength of the broad-band GLOBE detectors and for the spatiotemporal variation of ozone. Results from a theoretical error analysis indicate that GLOBE measurements achieve a precision better than 0.02 AOT for both channels. Comparisons with professional instruments generally give high correlations and low scatter and bias. From these tests, we conclude that student data are scientifically valid and may be used to validate MODIS AOT retrievals over the Netherlands. We find that over land, MODIS AOT is biased by +0.03 (470 nm) and -0.01 AOT (660 nm). Over coastal areas, MODIS overestimates AOT by 0.10 (470 nm) and 0.08 AOT (660 nm). Seasonally averaged MODIS observations over northwestern Europe show relatively highest AOT values over the region of Flanders and the Netherlands, with a seasonal cycle peaking in spring/summer. Our study shows the potential of secondary school-based networks in addition to existing, professional networks that have much less spatial coverage.