How can I compare two classes test scores fairly using a double bar graph?

Use a double bar graph with the same score ranges, same axis scale, and (if class sizes differ) percentages instead of raw counts. That way you compare like with like. 

Step 1. Group scores into the same ranges
Example ranges 0–20, 21–40, 41–60, 61–80, 81–100. 

Step 2. Count how many students fall in each range
If the classes have different strengths, convert counts to percentages.

Step 3. Plot a double bar for each range
For every score range, draw two bars side by side one for Class A and one for Class B. Use the same width and equal spacing. Label the x axis with the score ranges and the y axis with percent. Add a legend to show which colour is Class A and which is Class B. 

Step 4. Read it fairly
Compare bars within the same range only. Look for which class has taller bars in higher ranges. If needed, also report mean and median to summarise central performance and mention any outliers. This method shows a fair, clear picture of how the two classes performed across the whole score distribution.