For all schools, the correct arrangement of classes in ascending order of number of boys is ______.

The number of boys for each school is calculated by multiplying the Total Students by the Percentage of Boys.

School	Class IX Boys	Class X Boys	Class XI Boys	Class XII Boys
S1	$120 \times 40\% = 48$	$90 \times 30\% = 27$	$150 \times 60\% = 90$	$125 \times 80\% = 100$
S2	$150 \times 30\% = 45$	$100 \times 55\% = 55$	$120 \times 50\% = 60$	$100 \times 45\% = 45$
S3	$140 \times 20\% = 28$	$120 \times 40\% = 48$	$100 \times 65\% = 65$	$120 \times 60\% = 72$
S4	$100 \times 35\% = 35$	$100 \times 45\% = 45$	$80 \times 80\% = 64$	$150 \times 40\% = 60$
TOTAL	156	175	279	277

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The total number of boys in each class across all schools:

Class IX: $156$

Class X: $175$

Class XI: $279$

Class XII: $277$

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Arranging the classes from lowest to highest:

$156 < 175 < 277 < 279$

The correct arrangement is IX, X, XII, XI.