Hi there.
I think perhaps your graph looks something like this:
We have an online mathematics glossary that you can use to look up words like mode. It says:
M
mean
 in statistics, the measure of central tendency
calculated by adding all the values and dividing the sum by the
number of values. (Often referred to as the average.)
median
statistical
 the measure of central tendency that is in the
middle when the values are arranged in order of size. If there
is an even number of data items, the median is the mean of the middle two.
of a triangle
 a line segment from any vertex of a triangle
to the midpoint of the opposite side.

mode
 the value that appears most frequently in
a set of data.

So you are right. The most frequent grade is 1st Grade, so that is the mode.
The Median would be the middle value in the list when each person is listed in order of grade:
1. 
Kindergarten 


2. 
1st Grade 


3. 
1st Grade 


4. 
1st Grade 


5. 
1st Grade 


6. 
1st Grade 


7. 
2nd Grade 
← Median 
In a list of thirteen ordered elements, the seventh one is the median. 
8. 
2nd Grade 

9. 
2nd Grade 

10. 
2nd Grade 

11. 
3rd Grade 

12. 
3rd Grade 

13. 
3rd Grade 

The mean is what most people just call the "average", although sometimes we think of mean, median and mode as three kinds of average.
Hope this helps,
Stephen La Rocque.
