To find out how many students brought their lunch to school on Thursday, we can calculate 2% of the total number of students.
First, convert the percentage to a decimal: \[ 2% = \frac{2}{100} = 0.02 \]
Next, multiply this decimal by the total number of students: \[ 0.02 \times 900 = 18 \]
Therefore, the number of students who brought their lunch to school on Thursday is \( \boxed{18} \).