A stem-and-leaf is an abbreviation for a list of numbers, where they put
only the last digit of each number on the right of the "|" and put
all the digits but the last one of each number to the left of the |.

So the first line stands for the three numbers 126, 126, 127.
Since all three of those number begin with the digits "12", as a
short cut they put just one "12" before the | and then they put the
last digit of each number to the right of the |.  Notice that the "667"
to the right of the | does NOT mean "six hundred sixty-seven", although
that's what it looks exactly like.  But instead it means the three last
digits of three numbers beginning with "12".  They are 126, 126, and 127
listed on the first row below, under "Numbers meant by stem&leaf",  The
number 3 is under "Size" because there are three numbers.  The total
of 379 is gotten by adding 126+126+127 = 379.  At the bottom I added up
the sizes and got 40, which is how many numers there are altogether
represented.   I added up all the sums and got a grand sum at the bottom
of 2997 total students in all the sections.    
 

 	  Class        Numbers meant by stem&leaf     Size  Totals

 	     12|667           126, 126, 127             3     379
           11|89            118, 119                  2     237
           10|0257          100, 102, 105, 107        4     414
            9|1357           91,  93,  95,  97        4     376
            8|00377          80,  80,  83.  87, 87    5     417
            7|257            72,  75,  77             3     224
            6|22359          62,  62,  63,  65, 69    5     321
            5|3389           53,  53,  58,  59        4     223
            4|23366          42,  43,  43,  46, 46    5     220
            3|66789          36,  36,  37,  38, 39    5     186
 ---------------------------------------------------------------
                                                     40    2997


To get the mean, or average enrollment, you divide 2997 by 40 and get 

74.925 or about 75 on the average.

The median size enrollment is the average of the two numbers right in
the middle.  They are 75 and 77.  If you average them you get 76, so the 
median is 76.

You didn't state what you were asked to find besides the totals and the
grand total, so I just guessed that you wanted the mean and the median.

Edwin