There are 12 x 5/6 = <<12*5/6=10>>10 girls who are reading.
So, 12 - 10 = <<12-10=2>>2 girls are not reading.
There are 10 x 4/5 = <<10*4/5=8>>8 boys who are reading.
So, 10 - 8 = <<10-8=2>>2 boys are not reading.
Thus, there are a total of 2 girls + 2 boys = <<2+2=4>>4 students who are not reading.
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