If blindness is surveyed using schools as compared to population surveys, then estimation of prevalence of blindness will have ?
Blindness situation in India

Estimated prevalence of blindness in India (total) : 11.2 per 1000 population.
Estimated prevalence of blindness in India (0-14 years) : 0.1 per 1000 population.
Estimated prevalence of blindness in India (15-49 years) : 0.6 per 1000 population.
Estimated prevalence of blindness in India (50+ years) : 77.3 per 1000 population.
So if schools are used where only refractive errors generally constitute blindness (that too very few are actually blind i.e. <6 / 60) As compared to population (where age-related cataract constitute as major cause of blindness), it would lead to underestimation of prevalence of blindness in the country.
Overestimation
Underestimation
Remains same
None of them is used for evaluation