The school nurse must check a total of 26 Kindergarteners + 19 first graders + 20 second graders + 25 third graders = <<26+19+20+25=90>>90 students in all.
If every check takes 2 minutes, it will take 90 students x 2 minutes = <<90*2=180>>180 minutes.
There are 60 minutes in one hour, so it will take 180 minutes / 60 = <<180/60=3>>3 hours to complete the checks.
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