On arranging schools in descending order of total number of girls in all classes, the correct order is ______.

Since the table gives the percentage of boys, the percentage of girls is calculated as:

$\% \text{Girls} = 100\% - \% \text{Boys}$

The number of girls for each class is calculated using:

$\text{Number of Girls} = \text{Total Students} \times \% \text{Girls}$

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The table below breaks down the calculation for each class and sums the total girls for every school.

School	Class IX	Class X	Class XI	Class XII	Total Girls
S1	$120 \times 60\% = 72$	$90 \times 70\% = 63$	$150 \times 40\% = 60$	$125 \times 20\% = 25$	220
S2	$150 \times 70\% = 105$	$100 \times 45\% = 45$	$120 \times 50\% = 60$	$100 \times 55\% = 55$	265
S3	$140 \times 80\% = 112$	$120 \times 60\% = 72$	$100 \times 35\% = 35$	$120 \times 40\% = 48$	267
S4	$100 \times 65\% = 65$	$100 \times 55\% = 55$	$80 \times 20\% = 16$	$150 \times 60\% = 90$	226

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Arranging the schools from the highest number of girls to the lowest:

S3: 267

S2: 265

S4: 226

S1: 220

Therefore, the descending order is: $S3, S2, S4, S1$