First, find what the common denominator would be. To do this, you want to find the LCM of 12 and 10, or the smallest possible number that is divisible by both 12 and 10.
The area of the scale drawing would be 1080 square centimeters or (1080 cm^2). I got this answer through first finding the area of the classroom in feet (which is A= lw = [18][20] = 360 ft^2) and then I proceeded to converting it to centimeters via given scale factor (3 cm = 1 foot) and had the answer of 1080 centimeters squared.
